audio policy manager: fix BT SCO stream mute

Allow setting voice and bluetooth SCO stream volume to 0
in checkAndSetVolume() even when the selected device is
bluetooth SCO to allow muting VoIP call downlink.

Bug: 170489272
Test: repro steps in bug.
Change-Id: If8b2d225742f9589cd21718d8a439531996434f9
1 file changed