從Android N開始，原生系統增加了鎖屏桌布開關：

frameworks/base/packages/SystemUI/src/com/android/systemui/statusbar/phone/PhoneStatusBar.java

    /** If true, the lockscreen will show a distinct wallpaper */
    private static final boolean ENABLE_LOCKSCREEN_WALLPAPER = true;

要設定出廠預設鎖屏桌布，修改如下程式碼即可實現：
frameworks/base/packages/SystemUI/src/com/android/systemui/statusbar/phone/LockscreenWallpaper.java

    public Bitmap getBitmap() {
        if (mCached) {
            return mCache;
        }
        if (!mWallpaperManager.isWallpaperSupported()) {
            mCached = true;
            mCache = null;
            return null;
        }

        LoaderResult result = loadBitmap(mCurrentUserId, mSelectedUser);
        if
 
 (result.success) {
            mCached = true;
            mUpdateMonitor.setHasLockscreenWallpaper(result.bitmap != null);
            mCache = result.bitmap;
        }

    //add begin : 第一次開機載入失敗，嘗試讀取系統資源default_lock_wallpaper.jpg，然後寫入到鎖屏桌布檔案中，並重新loadBitmap
        if (mCache == null) {
            try
 
 {
                mWallpaperManager.setStream(
                        mContext.getResources().openRawResource(com.android.internal.R.drawable.default_lock_wallpaper),
                        null,
                        true,
                        WallpaperManager.FLAG_LOCK);

                result = loadBitmap(mCurrentUserId, mSelectedUser);
                if (result.success) {
                    mCached = true;
                    mUpdateMonitor.setHasLockscreenWallpaper(result.bitmap != null);
                    mCache = result.bitmap;
                }
            } catch (IOException e) {
                Log.e(TAG, "can not set default lockscreen wallpaper");
            }
        }
    //add end
        return mCache;
    }

然後放入系統資源圖片default_lock_wallpaper即可。