AudioService: Launch default music player on headset connect

This squashes the following commits from cm-13.0:
ffcfe4c5 AudioService: launch default music player on headset connect [2/3]
291b3914 Do not start music app when headset is unplugged
f87c5508 AudioService: don't launch music player while in call
95a503c4 AudioService: Prevent crash when there is no music app

Change-Id: Ibbb0607a56ab4d9246c14632eb4199558866854a