Add option to pass OverlayState for showing the fullscreen player

[SEGVeenstra]

This PR adds the overlay parameter to FlickVideoPlayer so you can provide an OverlayState that will be used to show the fullscreen player.

This resolves the issue where the fullscreen player only covers a portion of the screen if you are using nested navigators for example.

FlickVideoPlayer(
  flickManager: _manager!,
  overlay: Navigator.of(context, rootNavigator: true).overlay,
),

Small

Fullscreen without passing overlay

The bottom navigation remains visible

Fullscreen with passing overlay