SDK / exoplayer

Some Player implementations have no playlist capability but can still
set a MediaItem for playback. Examples are a MediaController connected
to a legacy MediaSession, ExoPlayer up to 2.12 or MediaPlayer.

To indicate this capability, we need an allowed command in addition
to COMMAND_CHANGE_MEDIA_ITEMS that just allows to set a single item
that replaces everything that is currently played.

#minor-release

PiperOrigin-RevId: 453879626
(cherry picked from commit 20561528)