Make MusicFX persistent service

This application must be running at all times, so that it can respond to
AudioEffect intents and manage audio sessions. Some users complained about
MusicFX not always managing their audio sessions, and I saw that
Android sometimes kills the MusicFX process and then ends up restarting
it mere seconds later. This is a likely cause for the problems, because
MusicFX will forget the IDs of the audio sessions in a restart.

Cherry-pick from 127dfea3e1b7c637321be0790ca667b2e28435f9

Change-Id: Icb8ef1f76fdebefa4d52bbcd95f0c4ddae4f50bb