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
b69ffd4dc2