AudioService: fix volume adjustment suppression

  Fix regression introduced by aosp/1202609 to allow rapid media
volume changes, which broke the ability to make changes to settings
(i.e. ringer mode) without changing the volume.
  The change consists in:
  a/ reverting regression
  b/ when changing MUSIC volume, if media is active, do not suppress
     the adjustement.
 (c/ document intended behavior)

Bug: 156010854
Test: see manual steps in bug
Change-Id: I3e6a3f55f09c8bd2bde01696298c145c53a8d321