audio policy: volume burst when switching device

The addition of the per device volume feature has introduced
a problem where a volume burst can be heard in the headphones
just after insertion if the speaker volume is much higher than the
headphones volume.

Added a temporary mute of the output when switching device to force
volume ramp to 0 and back up to new volume.

Issue 5984108.

Change-Id: I5c9ffbbcadd12d25c78cc2614d351346b8186c55
2 files changed