Implement Bluetooth device volume memory

Bluetooth volume memory is now handled on a per device basis. The volume
is stored in a shared preference and is loaded every time the device is
active. Volume is persisted for devices that both do and don't support
absolute volume.

Bug: 33962015
Test: Connect to multiple devices and switch between devices. See that
the volume is restored each time a device becomes active.

Change-Id: I596a4f0d701a7c5b8c52285807970f8b172cc423