VideoView cannot be attached outside of OnElementChanged in Xamarin.Forms

## 🐛 Bug Report

I have spent so much time trying to work around this.

I have a view that contains multiple VideoViews. I understand that I need to switch between them when this is the case by setting `CrossMediaManager.Current.MediaPlayer.VideoView = _videoView;`.

I've also set `CrossMediaManager.Current.MediaPlayer.AutoAttachVideoView = false;`

I made a `CustomVideoViewRenderer` so that I have control over the native `IVideoView`.
```
public class CustomVideoViewRendererAndroid : VideoViewRenderer
{
private MediaManager.Platforms.Android.Video.VideoView _videoView;

public CustomVideoViewRendererAndroid(Context context) : base(context)
{

}

protected override void OnElementChanged(ElementChangedEventArgs args)
{
if (args.OldElement != null)
{
args.OldElement.Dispose();
}

if (args.NewElement != null)
{
if (Control == null)
{
_videoView = new MediaManager.Platforms.Android.Video.VideoView(Context);
CrossMediaManager.Current.MediaPlayer.VideoView = _videoView;
SetNativeControl(_videoView);
UpdateBackgroundColor();
UpdateLayout();
}
}

base.OnElementChanged(args);
}

protected override void UpdateBackgroundColor()
{
base.UpdateBackgroundColor();
Control?.SetShutterBackgroundColor(Element.BackgroundColor.ToAndroid());
}

protected override void OnMeasure(int widthMeasureSpec, int heightMeasureSpec)
{
if (_videoView != null && widthMeasureSpec > -1 && heightMeasureSpec > -1)
{
var p = _videoView.LayoutParameters;

if (p == null)
p = new LayoutParams(widthMeasureSpec, heightMeasureSpec);
else
{
p.Height = heightMeasureSpec;
p.Width = widthMeasureSpec;
}
_videoView.LayoutParameters = p;
}
base.OnMeasure(widthMeasureSpec, heightMeasureSpec);
}

protected override void Dispose(bool disposing)
{
//On Android we need to call this manually it seems, since args.OldElement is never filled
Element?.Dispose();

_videoView = null;
base.Dispose(disposing);
}
}
```

This works. However, it's not what I need. Instead I need to set the current `VideoView` when it comes into frame. The calls all work and it all happens smoothly.

```
private void AttachChanged(object sender, bool e)
{
CrossMediaManager.Current.MediaPlayer.VideoView = _videoView;
}
```

This line is reached and executed fine. After this line I can inspect `CrossMediaManager.Current.MediaPlayer.VideoView` and see that it is indeed set to the `VideoView` that I want the controls to be linked to however the Controls do not apear on the page.

I truly hope that I'm missing something here. When I re-enable
`CrossMediaManager.Current.MediaPlayer.AutoAttachVideoView = true;` everything works again fine but the 2 VideoViews I am using are synced up and are attached to the same audio source it seems.

**Version:** 1.0.8

**Platform:**
- [ ] :iphone: iOS
- [ ] :robot: Android
- [ ] :checkered_flag: WPF
- [ ] :earth_americas: UWP
- [ ] :apple: MacOS
- [ ] :tv: tvOS
- [x] :monkey: Xamarin.Forms