audio policy: acquire lock for volume functions

Acquire the AudioPolicyService mutex when executing
setStreamVolumeIndex() and getStreamVolumeIndex().

These functions have become more complex and iterate on the
list of opened outputs and it is safer to acquire the
AudioPolicyService mutex to avoid concurrency with other
functions.

Bug 6626532.

Change-Id: Ifdcbbbf13338fd9a59f694d43faa7ade143f94bf