Add support for Picture-in-picture

Problem to solve

Native Picture-In-Picture support.

PiP mode can be in-app or external.

Intended users

Allow PiP for external-app video watching.

Documentation

• https://developer.apple.com/documentation/avkit/adopting_picture_in_picture_in_a_custom_player
• https://developer.android.com/guide/topics/ui/picture-in-picture
• https://developer.xamarin.com/samples/monotouch/ios9/PictureInPicture/
• https://github.com/escamoteur/RxMediaPlayer/blob/e405c6a14e839574654ded04a8d79ad9d59b36e5/Player.iOS/VideoView.cs