SDK / exoplayer

The MediaItemFiller is not flexible enough for most realworld usages
because:
 - it doesn't allow asynchronous resolution of MediaItems (e.g. to
   look up URIs from a database)
 - it doesn't allow to batch updates for multiple items or do more
   advanced customizations (e.g. expanding a mediaId representing
   a playlist to multiple items).

Both issues can be solved by passing in a list of items and
returning a ListenableFuture. The callback itself can also move
into MediaSession.Callback for consistency with the other
callbacks.

PiperOrigin-RevId: 451857319