Implement AutomaticKeepAliveClientMixin

Hi, I think `ChewieState` should implement `AutomaticKeepAliveClientMixin` with sufficient logic to keep this widget alive with ancestors such as SliverList/ListView (for example, checking for value.initialized == true). At least, it would be extremely useful to have an opt-in flag to avoid using custom wrappers just to do this.

I even think that audio playback should continue event without KeepAlive if the `VideoPlayerController` isn't disposed (and video resumed when the widget is re-attached), but that's probably impossible to resolve at Chewie level.