AudioService: Write base stream volume changes to the event log.

 - Add a new volume_changed event, reported at the stream level.
 - Only include changes to base streams (no aliases).
 - Include the caller for each change.  A caller is either:
   -  a pkg name (for external calls or known media sessions)
   -  a system server class's log tag (for internal calls,
      disambiguates "android")

Bug: 19599935
Change-Id: Ia61b68ff1e7e2907a24972790ec052bfe099e665