AudioService: Observe changes to output devices per-stream.

 - Keep track of calls to AudioSystem.getDevicesForStream.
 - Report device changes for the base streams to the binary event log.
 - Fire internal intent so volume UI can respond to changes.
 - Update devices when new routes are reported.

Bug: 19618959
Change-Id: I7bbd497b178d9829accc19e6c951a711aef8590c