Playlist can now be obtained directly from Timeline windows
in any state. So make ExoPlayer as the trusted source of
playlist, instead of SessionPlayerConnector.
PlayerWrapper still need to keep the list of media items.
It's used to detect whether the Timeline change is caused by
changes in media items or not, and only notify
SessionPlayer.PlayerCallback#onPlaylistChanged() only when
the playlist is really changed.
PiperOrigin-RevId: 327231820
