SDK / exoplayer

These legacy callbacks are currently forwarded to onSetMediaUri which
will be removed in the future.

Also make sure to only call player.prepare/play after the items have
been set.

The calls to onAddQueueItem are also forwarded to onAddMediaItems to
actually allow a session to resolve these items to playable media, which
wasn't possible so far.

PiperOrigin-RevId: 453625204
(cherry picked from commit bd126ec5)