Android 横屏设置, 为什么会先显示一会竖屏

Actiivty 横屏设置

    var TAG = "Test"

    override fun onCreate(savedInstanceState: Bundle?) {
        setOrientation()
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        Log.d(TAG, "onCreate")
    }

    private fun setOrientation() {
        //隐藏标题
        requestWindowFeature(Window.FEATURE_NO_TITLE);
        //设置全屏
        window.setFlags(
            WindowManager.LayoutParams.FLAG_FULLSCREEN,
            WindowManager.LayoutParams.FLAG_FULLSCREEN
        )
        requestedOrientation = ActivityInfo.SCREEN_ORIENTATION_LANDSCAPE
    }

为什么会先出来一会的竖屏, 才会横屏显示 ?

但是在 AndroidManifest.xml 中设置 

就能正常横屏显示....