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 5333c67d)
