Improve audio service.

* Add support for remote clients, i. e. lockscreen controls.
* Add support for system audio focus.
Add audio player notification for foreground behaviour.
Change audio file viewer to use metadata fetcher..
Fix audio viewer to extt without hiding controller first.
Fix recycling audio art by remote.
Add possible freezes detection.
Refactor audio viewer related code.
Merge branch 'master' into audio-service
release the audio improvements as 0.1.4