Fix NullPointerException in System UI caused by remote volume change

VolumeDialogController(System UI) only manages active sessions,
but MediaSessionService always notifies the System UI of remote
volume change regardless of whether the session is active or not.
This CL makes MediaSessionService notify of remote volume change
only when the session is active.

Bug: 34033926
Test: Flashed new master build on angler and tested.
Change-Id: Id363c078f75d49732aa7af9578ee44a24649df63