Store volume when the active device is set to null

Store volume device when setting active device to null and move AVRCP
volume memory code into removeActiveDevice.
Add method in ServiceFactory to get AvrcpTargetSercie for unit test to
mock AvrcpTargetService.

Bug: 128471448
Bug: 124818780
Test: runtest bluetooth
Change-Id: Icf5d5f00c04436038cf0790a0f7bfd81cb62a451