AvrcpTarget: Fix not unregistering AudioDeviceCallback

When AvrcpTargetService is destroyed, the AvrcpVolumeManager instance
inside the service did not unregister the callback.

Bug: 381431423
Test: atest BluetoothInstrumentationTests
Flag: EXEMPT; call sequence verified with unit test
Change-Id: I59d0bb0b8a9567672fa4e0525157791aba9fb43e