Improvement for issue 3489986: BT SCO volume

This change makes sure that the VOICE_CALL stream volume tracks
the BLUETOOTH_SCO stream volume when SCO audio is enabled.
The down link audio volume now reflects what is being displayed
when pressing volume hard keys on the device while in a video chat
with a BT SCO headset.

Volume settings on the headset and the device are still independent as
we do not support handsfree profile yet.

Change-Id: Ie0d2714730ea359b9318b9cbe6f0b2557ef0f976