Fix A2DP volume at 0 on connection

A regression on A2DP volume was introduced in the rebase
  of 5885037c
  on top of 9fbc205f
  where the uninitialized arg2 field of the message was
  used as the A2DP volume, whereas the original CL was
  storing the volume information in the BluetoothA2dpDeviceInfo
  object.

Bug: 123554971
Test: play music on speaker, connect A2DP device, verify music not muted
Change-Id: Id14f4c93358cd539ef51d00c806a1562bd6f0008