SDK / exoplayer

Being specific, this includes following changes
  - Remove PlaylistManager and TimelinePlaylistManager
    and use Player's playlist API directly.
  - Replace ConcatenatingMediaSource uses with
    ExoPlayer MediaItem.
  - Replace PlaybackPreparer uses with Player#prepare()
  - Add MediaItemConverter for developers to customize
    converting AndroidX MediaItems to ExoPlayer MediaItems
    and vice-versa.
  - Add DefaultMediaItemConverter for providing default
    implementation of both MediaItemConverter
    and MediaSourceFactory.

Note that removing PlaylistManager loses the ability
to suppress individual playlist API. But decided to remove
for simpler API set. The feature can be added back later
via explicit request.

PiperOrigin-RevId: 326463492