This fixes an inconsistent state of the `PlayerInfo` when the index of the playing
media item is changed by a playlist modification. In this inconsistent state,
calling `Playerinfo.getCurrentMediaItem` can produce an
`ArrayIndexOutOfBoundException` (see stack trace in GH issue).
This change takes the following measurements:
- always update sessionPosition and timeline of the PlayerInfo together in
`MediaSessionImpl.PlayerListener` where the PlayerInfo originates from
- add an assertion to avoid building a `PlayerInfo` instance in an inconsistent
state
- reduce the window of opportunity for concurrent access to
`mediaSessionImpl.playerInfo` when dispatching player info changes in
`MediaSessionImpl`
Issue: androidx/media#51
PiperOrigin-RevId: 444812661
