三個虛擬按鍵的設定是在framework層中。
    先檢視三個按鍵的佈局的初始化：
位置 frameworks/base/packages/SystemUI/src/com/android/systemui/statusbar/phone/PhoneStatusBar.java:
protected PhoneStatusBarView makeStatusBarView() {
if (showNav) {
                mNavigationBarView =
                    (NavigationBarView) View.inflate(context, R.layout.navigation_bar, null);

                mNavigationBarView.setDisabledFlags(mDisabled);
                mNavigationBarView.setBar(this);
                mNavigationBarView.setOnTouchListener(new View.OnTouchListener() {
                    @Override
                    public boolean onTouch(View v, MotionEvent event) {
                        checkUserAutohide(v, event);
                        return false;
                    }});
            }

}
    在上面的程式碼中，mNavigationBarView = (NavigationBarView) View.inflate(context, R.layout.navigation_bar, null），該程式碼動態載入了該佈局。常看該佈局的程式碼：
位置 frameworks/base/packages/SystemUI/res/layout/navigation_bar.xml：
<com.android.systemui.statusbar.phone.NavigationBarView
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:systemui="http://schemas.android.com/apk/res/com.android.systemui"
    android:layout_height="match_parent"
    android:layout_width="match_parent"
    android:background="@drawable/system_bar_background"
    >
<!--
 -->
    <FrameLayout android:id="@+id/rot0"
        android:layout_height="match_parent"
        android:layout_width="match_parent"
        >

        <LinearLayout
            android:layout_height="match_parent"
            android:layout_width="match_parent"
            android:orientation="horizontal"
            android:clipChildren="false"
            android:clipToPadding="false"
            android:id="@+id/nav_buttons"
            android:animateLayoutChanges="true"
            >

            <!-- navigation controls -->
            <View
                android:layout_width="40dp"
                android:layout_height="match_parent"
                android:layout_weight="0"
                android:visibility="invisible"
                />
            <com.android.systemui.statusbar.policy.KeyButtonView android:id="@+id/back"
                android:layout_width="@dimen/navigation_key_width"
                android:layout_height="match_parent"
                android:src="@drawable/ic_sysbar_back"
                systemui:keyCode="4"
                android:layout_weight="0"
                android:scaleType="center"
                systemui:glowBackground="@drawable/ic_sysbar_highlight"
                android:contentDescription="@string/accessibility_back"
                />
            <View
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:layout_weight="1"
                android:visibility="invisible"
                />
            <com.android.systemui.statusbar.policy.KeyButtonView android:id="@+id/home"
                android:layout_width="@dimen/navigation_key_width"
                android:layout_height="match_parent"
                android:src="@drawable/ic_sysbar_home"
                systemui:keyCode="3"
                systemui:keyRepeat="false"
                android:layout_weight="0"
                systemui:glowBackground="@drawable/ic_sysbar_highlight"
                android:contentDescription="@string/accessibility_home"
                />
            <View
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:layout_weight="1"
                android:visibility="invisible"
                />
            <com.android.systemui.statusbar.policy.KeyButtonView android:id="@+id/recent_apps"
                android:layout_width="@dimen/navigation_key_width"
                android:layout_height="match_parent"
                android:src="@drawable/ic_sysbar_recent"
                android:layout_weight="0"
                systemui:glowBackground="@drawable/ic_sysbar_highlight"
                android:contentDescription="@string/accessibility_recent"
                />
            <com.android.systemui.statusbar.policy.KeyButtonView android:id="@+id/menu"
                android:layout_width="@dimen/navigation_menu_key_width"
                android:layout_height="match_parent"
                android:src="@drawable/ic_sysbar_menu"
                systemui:keyCode="82"
                android:layout_weight="0"
                android:visibility="invisible"
                android:contentDescription="@string/accessibility_menu"
                systemui:glowBackground="@drawable/ic_sysbar_highlight"
                />
        </LinearLayout>

        <!-- lights out layout to match exactly -->
        <LinearLayout
            android:layout_height="match_parent"
            android:layout_width="match_parent"
            android:orientation="horizontal"
            android:id="@+id/lights_out"
            android:visibility="gone"
            >
            <ImageView
                android:layout_width="80dp"
                android:layout_height="match_parent"
                android:layout_marginStart="40dp"
                android:src="@drawable/ic_sysbar_lights_out_dot_small"
                android:scaleType="center"
                android:layout_weight="0"
                />
            <View
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:layout_weight="1"
                android:visibility="invisible"
                />
            <ImageView
                android:layout_width="80dp"
                android:layout_height="match_parent"
                android:src="@drawable/ic_sysbar_lights_out_dot_large"
                android:scaleType="center"
                android:layout_weight="0"
                />
            <View
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:layout_weight="1"
                android:visibility="invisible"
                />
            <ImageView
                android:layout_width="80dp"
                android:layout_marginEnd="40dp"
                android:layout_height="match_parent"
                android:src="@drawable/ic_sysbar_lights_out_dot_small"
                android:scaleType="center"
                android:layout_weight="0"
                />
        </LinearLayout>

        <FrameLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent">

            <com.android.systemui.statusbar.policy.KeyButtonView
                android:layout_width="80dp"
                android:id="@+id/search_light"
                android:layout_height="match_parent"
                android:layout_gravity="center"
                android:src="@drawable/search_light"
                android:scaleType="center"
                android:visibility="gone"
                android:contentDescription="@string/accessibility_search_light"
                />

            <com.android.systemui.statusbar.policy.KeyButtonView
                android:id="@+id/camera_button"
                android:layout_height="match_parent"
                android:layout_width="80dp"
                android:layout_gravity="center_vertical|right"
                android:src="@drawable/ic_sysbar_camera"
                android:scaleType="center"
                android:visibility="gone"
                android:contentDescription="@string/accessibility_camera_button"
                />
        </FrameLayout>

        <com.android.systemui.statusbar.policy.DeadZone
            android:id="@+id/deadzone"
            android:layout_height="match_parent"
            android:layout_width="match_parent"
            systemui:minSize="@dimen/navigation_bar_deadzone_size"
            systemui:maxSize="@dimen/navigation_bar_deadzone_size_max"
            systemui:holdTime="@integer/navigation_bar_deadzone_hold"
            systemui:decayTime="@integer/navigation_bar_deadzone_decay"
            systemui:orientation="horizontal"
            android:layout_gravity="top"
            />
    </FrameLayout>

    <FrameLayout android:id="@+id/rot90"
        android:layout_height="match_parent"
        android:layout_width="match_parent"
        android:visibility="gone"
        android:paddingTop="0dp"
        >

        <LinearLayout
            android:layout_height="match_parent"
            android:layout_width="match_parent"
            android:orientation="vertical"
            android:clipChildren="false"
            android:clipToPadding="false"
            android:id="@+id/nav_buttons"
            android:animateLayoutChanges="true"
            >

            <!-- navigation controls -->
            <com.android.systemui.statusbar.policy.KeyButtonView android:id="@+id/menu"
                android:layout_height="40dp"
                android:layout_width="match_parent"
                android:src="@drawable/ic_sysbar_menu_land"
                systemui:keyCode="82"
                android:layout_weight="0"
                android:visibility="invisible"
                android:contentDescription="@string/accessibility_menu"
                systemui:glowBackground="@drawable/ic_sysbar_highlight_land"
                />
            <com.android.systemui.statusbar.policy.KeyButtonView android:id="@+id/recent_apps"
                android:layout_height="80dp"
                android:layout_width="match_parent"
                android:src="@drawable/ic_sysbar_recent_land"
                android:layout_weight="0"
                android:contentDescription="@string/accessibility_recent"
                systemui:glowBackground="@drawable/ic_sysbar_highlight_land"
                />
            <View
                android:layout_height="match_parent"
                android:layout_width="match_parent"
                android:layout_weight="1"
                android:visibility="invisible"
                />
            <com.android.systemui.statusbar.policy.KeyButtonView android:id="@+id/home"
                android:layout_height="80dp"
                android:layout_width="match_parent"
                android:src="@drawable/ic_sysbar_home_land"
                systemui:keyCode="3"
                systemui:keyRepeat="false"
                android:layout_weight="0"
                android:contentDescription="@string/accessibility_home"
                systemui:glowBackground="@drawable/ic_sysbar_highlight_land"
                />
            <View
                android:layout_height="match_parent"
                android:layout_width="match_parent"
                android:layout_weight="1"
                android:visibility="invisible"
                />
            <com.android.systemui.statusbar.policy.KeyButtonView android:id="@+id/back"
                android:layout_height="80dp"
                android:layout_width="match_parent"
                android:src="@drawable/ic_sysbar_back_land"
                android:scaleType="center"
                systemui:keyCode="4"
                android:layout_weight="0"
                android:contentDescription="@string/accessibility_back"
                systemui:glowBackground="@drawable/ic_sysbar_highlight_land"
                />
            <View
                android:layout_height="40dp"
                android:layout_width="match_parent"
                android:layout_weight="0"
                android:visibility="invisible"
                />
        </LinearLayout>

        <!-- lights out layout to match exactly -->
        <LinearLayout
            android:layout_height="match_parent"
            android:layout_width="match_parent"
            android:orientation="vertical"
            android:id="@+id/lights_out"
            android:visibility="gone"
            >
            <ImageView
                android:layout_height="80dp"
                android:layout_marginTop="40dp"
                android:layout_width="match_parent"
                android:src="@drawable/ic_sysbar_lights_out_dot_small"
                android:scaleType="center"
                android:layout_weight="0"
                />
            <View
                android:layout_height="match_parent"
                android:layout_width="match_parent"
                android:layout_weight="1"
                android:visibility="invisible"
                />
            <ImageView
                android:layout_height="80dp"
                android:layout_width="match_parent"
                android:src="@drawable/ic_sysbar_lights_out_dot_large"
                android:scaleType="center"
                android:layout_weight="0"
                />
            <View
                android:layout_height="match_parent"
                android:layout_width="match_parent"
                android:layout_weight="1"
                android:visibility="invisible"
                />
            <ImageView
                android:layout_height="80dp"
                android:layout_marginBottom="40dp"
                android:layout_width="match_parent"
                android:src="@drawable/ic_sysbar_lights_out_dot_small"
                android:scaleType="center"
                android:layout_weight="0"
                />
        </LinearLayout>

        <com.android.systemui.statusbar.policy.KeyButtonView
            android:id="@+id/search_light"
            android:layout_height="80dp"
            android:layout_width="match_parent"
            android:layout_gravity="center_vertical"
            android:src="@drawable/search_light_land"
            android:scaleType="center"
            android:visibility="gone"
            android:contentDescription="@string/accessibility_search_light"
            />

        <!-- No camera button in landscape mode -->

        <com.android.systemui.statusbar.policy.DeadZone
            android:id="@+id/deadzone"
            android:layout_height="match_parent"
            android:layout_width="match_parent"
            systemui:minSize="@dimen/navigation_bar_deadzone_size"
            systemui:maxSize="@dimen/navigation_bar_deadzone_size_max"
            systemui:holdTime="@integer/navigation_bar_deadzone_hold"
            systemui:decayTime="@integer/navigation_bar_deadzone_decay"
            systemui:orientation="vertical"
            android:layout_gravity="top"
            />
    </FrameLayout>

    <!-- not used -->
    <View android:id="@+id/rot270"
        android:layout_height="match_parent"
        android:layout_width="match_parent"
        android:visibility="gone"
        />

</com.android.systemui.statusbar.phone.NavigationBarView>
其中，android:background="@drawable/system_bar_background"該屬性用於設定佈局的背景（個人嘗試修改該屬性，貌似沒有起作用）
@drawable/system_bar_background，背景圖片就是一種顏色
位置：frameworks/base/packages/SystemUI/res/values/colors.xml
<drawable name="system_bar_background">@color/system_bar_background_opaque</drawable>
<color name="system_bar_background_opaque">#77000000</color>

進入NavigationBarView的原始碼中檢視：
位置：frameworks/base/packages/SystemUI/src/com/android/systemui/statusbar/phone/NavigationBarView.java
 public NavigationBarView(Context context, AttributeSet attrs) {
        super(context, attrs);

        mDisplay = ((WindowManager)context.getSystemService(
                Context.WINDOW_SERVICE)).getDefaultDisplay();

        final Resources res = mContext.getResources();
        mBarSize = res.getDimensionPixelSize(R.dimen.navigation_bar_size);
        mVertical = false;
        mShowMenu = false;
        mDelegateHelper = new DelegateViewHelper(this);

        getIcons(res);

        mBarTransitions = new NavigationBarTransitions(this);

        mCameraDisabledByDpm = isCameraDisabledByDpm();
        watchForDevicePolicyChanges();
    }
其中，mBarTransitions = new NavigationBarTransitions(this)中例項化了一個大用於佈局的過渡的例項，按鍵佈局作為引數傳遞到該類中，檢視NavigationBarTransitions類的原始碼：
位置：frameworks/base/packages/SystemUI/src/com/android/systemui/statusbar/phone/NavigationBarTransitions.java
 public NavigationBarTransitions(NavigationBarView view) {
        super(view, R.drawable.nav_background);
        mView = view;
        mBarService = IStatusBarService.Stub.asInterface(
                ServiceManager.getService(Context.STATUS_BAR_SERVICE));
    }

    看super(view, R.drawable.nav_background)程式碼，可以知道該類的建構函式呼叫了其父類的建構函式，其中，NavigationBarView例項和一個圖片的Id作為引數傳到該類的父類的建構函式中，進入NavigationBarTransitions類的父類，檢視程式碼：
位置：frameworks/base/packages/SystemUI/src/com/android/systemui/statusbar/phone/BarTransitions.java

public BarTransitions(View view, int gradientResourceId) {
        mTag = "BarTransitions." + view.getClass().getSimpleName();
        mView = view;
        mBarBackground = new BarBackgroundDrawable(mView.getContext(), gradientResourceId);
        if (HIGH_END) {
            Log.i("Garment27", "BarTransitions---HIGH_END");
            mView.setBackground(mBarBackground);
        }
    }

從其建構函式可以看到，mView.setBackground(mBarBackground)這段程式碼就是把傳入的圖片設定成為按鍵佈局的背景（在Launcher3中，該圖片是一張透明的圖片，所以得到的背景是透明的）
嘗試把mView.setBackground(mBarBackground)這段程式碼遮蔽了，然後按鍵佈局就只顯示<color name="system_bar_background_opaque">#77000000</color>中定義的顏色，上面的statusbar和下面的虛擬按鍵佈局的背景設定是共用一套的背景，是因為兩個佈局都是有一個過渡類，大家都是繼承BarTransitions類。

其實，虛擬按鍵佈局的背景顏色是會有一個過度的，在Launcher介面中，虛擬按鍵佈局背景圖是顯示的是透明的圖片，當我們進入到某一個應用中的時候，背景圖就會過渡為黑色的背景。其中的程式碼是如何實現的，還有待研究。