Clear A2DP active device during start/stop in the native stack

Use setActiveDevice(null) during A2dpService.start() and stop()
to clear the A2DP active device both in A2dpService and the native
Bluetooth stack.
Otherwise, if A2dpService is restarted without the Bluetooth process
itself being restarted, the native Bluetooth stack might contain
stale state about the active device.

Bug: 73832547
Test: Manual - A2DP streaming, enable Location -> Bluetooth scanning,
      restart Bluetooth in Quick Settings
Change-Id: I8e001215ef052e588ddf6c53e2caf1382a70ae9b