AudioService: fix BT SCO volume mute

BT SCO stream should not be automatically muted when its index reaches 0.
It can only be muted via the adjustStreamVolume API with mute direction.

Bug: 267184792
Test: repro steps in bug
Test: atest AudioManagerTest
Test: atest AccessibilityVolumeTest
Test: atest AudioServiceHostTest#testAudioVolumeGroups

Change-Id: I55441ca4947a24cf1488538ecfb3222d79bc5f49