Unregister callback in background thread.

Unregistering callback for media controller is blocking the main thread.
This CL sends the unregistering execution to background.

Flag: NONE
Bug: 336612071
Test: Posted media notification and removed it many times. Main thread
doesn't seem to be blocked.

Change-Id: Ifb3ceab533c40c2e0da7b6e72cc0a27776d2e6be