audio policy: mute call RX when SCO volume is 0

Set voice volume to 0 when SCO volume index is 0 to
allow muting RX audio. when index is not 0, voice volume
is set to max to allow volume control by headset as before.

Bug: 111195322
Test: check call volume with SCO headset
Change-Id: I90466b01a9899929cd3e6fb1ac0a4985099497fc