Refactor for audio focus, remote control, media button handling

Extract audio focus, remote control and media button handling
 outside of AudioService without any changes in functionality.
 Moving logic to new class, MediaFocusControl.
Introduce interface for managing volum control logic, VolumeController.
 The VolumePanel class implements this interface.

Change-Id: I72bda2e0670c26e61ff076fd729c15f9f1156dc5