Ensure only one player view is in the widget tree at a time

More low-level video player implementations, like my [`omxplayer_video_player`](https://github.com/ardera/omxplayer_video_player) implement video playback using platform views, with one video player having _at most 1 platform view_ at a time. That is ensured by giving the Widget built by [`VideoPlayerPlatform.buildView`](https://pub.dev/documentation/video_player_platform_interface/latest/video_player_platform_interface/VideoPlayerPlatform/buildView.html) a [`GlobalObjectKey`](https://api.flutter.dev/flutter/widgets/GlobalObjectKey-class.html) with the textureId as its identity value. (Actually, not GlobalObjectKey itself, but a private descendant of GlobalObjectKey)

Chewie seems to sometimes have 2 video player views in the widget tree at the same time. (Because I get corresponding errors by GlobalObjectKey).

My proposal is to modify the chewie so it displays at most one video view for a player at a time.

This is necessary so `omxplayer_video_player` can run flawlessly, but it also likely increases performance for other video player implementations.