AudioService: fix cross deadlock in VolumeStreamState

Synchronize modifications of volume index by VolumeStreamState
class mutex instead of using synchronized methods.
This avoids possible cross deadlock when modifying volume on
two stream types simultaneously and one is slave to the other.

Bug: 13730145.

Change-Id: I13406c71010ce0c2e2f08f660b6101f310396c98