Android floating window on flutter?

I need to draw a small floating window outside of the app. Something like facebooks floating avatars on top of the screen.

Here is my service:

@RequiresApi(Build.VERSION_CODES.O)
class FloatingWindowService : Service() {
  private lateinit var windowManager: WindowManager

  override fun onCreate() {
      super.onCreate()
      windowManager = getSystemService(Context.WINDOW_SERVICE) as WindowManager

      val params = WindowManager.LayoutParams(
            WindowManager.LayoutParams.WRAP_CONTENT,
            WindowManager.LayoutParams.WRAP_CONTENT,
            WindowManager.LayoutParams.TYPE_APPLICATION_OVERLAY,
            WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE,
            PixelFormat.TRANSLUCENT
        )
    windowManager.addView(/** here shuld be the view controlled by flutter **/, params)
  }
}

Any ideas about how to do this?