Vol key presses: active stream has priority of set control stream

When deciding which volume stream is to be controlled by key
  presses, give priority to the stream that is actually playing,
  unless the user has selected a another volume to be controlled
  by the key presses.

Test: play some music over headset, pause, go to home screen.
  Press vol up, it changes ringer volume, immediately press play
  and press vol up again, it should now change music volume
  (and UI updates to indicate that).
Bug: 33355229

Change-Id: Ibad9b59a692e1c6375487b739c55fdbf07f107dd