HowTo render offscreen and then onscreen with ExoPlayer

Question

I am using ExoPlayer with Xamarin Android.

What I want:

• create ExoPlayer from Code and load video offscreen
• render frist frame offscreen and pause
• capture bitmap of that first frame offscreen
• load bitmap to ImageView onscreen
• hide ImageView and resume video playback onscreen

exoPlayer = new SimpleExoPlayer.Builder(UI.Player.Context).Build();
var listener = new PlayerListener();
listener.init(ExoPlayer_RenderedFirstFrame); // my callback method
exoPlayer.AddListener(listener);
exoPlayer.AddVideoListener(listener);

// Draw offscreen in surface
GLES30.GlEnable(GLES11Ext.GlTextureExternalOes);
GLES30.GlGenTextures(1, _textures, 0);
GLES30.GlBindTexture(GLES11Ext.GlTextureExternalOes, _textures[0]);

GLES30.GlBindTexture(GLES11Ext.GlTextureExternalOes, 0);
surfaceTexture = new SurfaceTexture(_textures[0], true);
surface = new Surface(surfaceTexture);


exoPlayer.SetVideoSurface(surface);
exoPlayer.Prepare(buildMediaSourceNew(videoUri));
exoPlayer.PlayWhenReady = true;


 private void ExoPlayer_RenderedFirstFrame()
            { 
exoPlayer.PlayWhenReady = false;
                usePixelCopy(surface, pixelCopyCallback);
            }

 public void pixelCopyCallback(Bitmap bitmap)
            {              
                UI.Player.Context.RunOnUiThread(() =>
                {
                    var drawable = new BitmapDrawable(UI.Player.Context.Resources, bitmap);
                    drawable.SetAntiAlias(true);
                    imageFrame.SetImageDrawable(drawable);
                });
            }

The above code is working. But how do I redirect the rendering to the UI? Surface is not a UI control. I was not able to find a way to add the existing surface or SurfaceTexture to the UI :-(