Before this change, media button events are routed from `onStartCommand`
of the `MediaSessionService` to the `MediaSessionCompat`, resolved by
the legacy library to a session command called on
`MediaSessionCompat.Callback` from where the command is delegated back
to the Media3 session.
With this change the keycode is resolved directly to a Media3 command
that is sent to the session through the media notification controller
of the session.
After this change, a playback or custom command sent to the session
from a notification, either as a pending intent (before API 33) or as
a legacy session command, look the same and the caller is the
media notification controller on all API levels.
PiperOrigin-RevId: 568224123
ffd7bb5639