audio: Volume can't adjust during HFP call when screen is off.

PhoneWindowManager don't pass the key to the application when screen is
off. In handleVolumeKey(), the STREAM_VOICE_CALL volume was set, so the
STREAM_BLUETOOTH_SCO volume can't be adjusted.
Add a judgement to choose the correct stream to adjust when screen is off.

CRs-Fixed: 649548

Change-Id: I1ab06617d59b9bf2eabed9ed8b6892bc20db75b9