Use an observer for longpressback to kill

Change-Id: Ibd4e1636a430fcf5cca4ea0e95fb3c62a2d5b42f
VanirAwesome: only get an audiomanager for sound-related actions

and print some stuff while doing last_app action for debugging it

Change-Id: Iccc49aa3314f9fe09bad4fb92394fd78aa3c11f6