audiopolicy: Constrain session events to music streams

 * We're really only interested in music streams right now, but
   events are being generated for all streams (system sounds, etc).
 * Constrain for now, in the future we will filter based on client
   registrations.

Ticket: CYNGNOS-3219

Change-Id: Ic445052028c454eed146addebcdb28c4b26c4f20
(cherry picked from commit a5ec3545)