Don't set Bluetooth volume when the Bluetooth stream changes

Don't restore the remembered volume whenever an Bluetooth A2DP stream
comes up or whenever Bluetooth AVRCP reports absolute volume support.
All Bluetooth volume memory will now be handled by Bluetooth.

Bug: 30602783
Test: See that AudioService doesn't try to change the volume when a
Bluetooth device connects.

Change-Id: I683062c1a0565591ae33cbbb0381730b86ad17c2