audio service: fix volume burst on user switch

Change the convention on use of default device volume with
audio policy manager: Now setting the default device volume on a stream does not
reset all specific device volumes. The default volume is just used by audio policy
manager if no specific device volume is present for a given selected device.

Bug: 27557733

Change-Id: I1edd9530ccafc615d9353b9b06489c688e21f719