Store audible stream volume if A2DP device disconnets in mute state.

Bluetooth stores an incorrect volume index if A2DP device disconnects in mute
state. So after the A2DP device is reconnected, the incorrect volume
index returns to the Bluetooth.

Bug: 158252625

Test: Connect the A2DP device, and press "Volume up" key, record the volume
index. Press "mute" key. Turn off A2DP device and then system will
disconnect the A2DP device. And then turn on A2DP device and the system will
automatically reconnect the device. Press "Volume up" key and check the volume
index.

Change-Id: I55a360f7ed3c8db712b4f9ccd636f073c37532e4