audiopolicy: rework stream activity for volume management on output

-Manage duplicated output ref count in client itself
-Use VolumeSource to track volume activity in output
(today mapped on legacy stream type)
-Use accessor APIs to control activity/mute from apm.

Test: audio smoke tests
Bug: 124767636

Change-Id: I452e3973f6869d41231d984896a5886ebb86afb1
Signed-off-by: François Gaffie <francois.gaffie@renault.com>