The deprecated `Player.addListener(EventListener)`
is moved out of Player into its subclasses
(CastPlayer and ExoPlayer).
This is unlikely to break users because:
- the method has been deprecated in the last major version
- the method is still present in the major implementations
If an users is affected, they can either:
- use ExoPlayer instead of Player
- (recommended) switch to Player.Listener.
Additionally update the threading guarantees that did not
reflect the current implementation.
PiperOrigin-RevId: 397272144
74c6ef9ba0