AudioService: fix volume group mute

Make sure the volume applied to audio policy by a volume group follows
the mute state of the associated public stream if any.

Bug: 156559315
Test: repro steps in bug
Test: atest AudioManagerTest#testVolume
Test: atest AudioVolumeGroupTest

Change-Id: I95e96f3524b27600c59a2cf093cd3309430d28ca