Minimum work to make volume handling work with sessions

This is the minimum change to make adjusting volume work with
MediaSessions. This only affects adjusting the volume and adjusting
the volume with a suggested stream. Adjusting a specific stream or
setting a specific stream will still use the same code.

This does not fix existing remote volume handling in RCC, which
will require a separate change to MediaController.

Change-Id: I5b957ff4bece1ee11e2364e1f216e1c08343c983