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
