This change adds `MediaController.getSessionExtras()` through
which a controller can access the session extras.
The session extras can be set for the entire session when
building the session. This can be overridden for specific
controllers in `MediaSession.Callback.onConnect`.
PiperOrigin-RevId: 584430419
(cherry picked from commit a063d137b4307348a140ec6a2b6d254db294395e)