AudioService: VOICE_CALL vol changes map to SCO in calls

During a call over BT SCO, volume is controlled by
STREAM_BLUETOOTH_SCO. But applications shouldn't have to know
about the routing to control the volume.
Remap STREAM_VOICE_CALL to STREAM_BLUETOOTH_SCO in
setStreamVolume during a call over SCO.

Bug: 326861992
Test: place call on SCO, adb shell cmd audio set-volume 0 15 (0 for
STREAM_VOICE_CALL followed by vol index)

Change-Id: Ib5abee36380cc6cab8579044f105e0d503397fd5