在setContentView之前新增：

override fun onCreate(savedInstanceState: Bundle?) {
    window.requestFeature(Window.FEATURE_NO_TITLE)
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
        val window = window
        window.clearFlags(WindowManager.LayoutParams.FLAG_TRANSLUCENT_STATUS or WindowManager.LayoutParams.FLAG_TRANSLUCENT_NAVIGATION)
        window.decorView.systemUiVisibility = View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN or View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION or View.SYSTEM_UI_FLAG_LAYOUT_STABLE
        window.addFlags(WindowManager.LayoutParams.FLAG_DRAWS_SYSTEM_BAR_BACKGROUNDS)
        window.statusBarColor = Color.TRANSPARENT
        window.navigationBarColor = Color.TRANSPARENT
    }