android: Main is now a service

- add Settings: Activity to start / stop MPD Service (Main).

- Main is a service that run in foreground with a notification. See
  Service.startForeground documentation for more details.

- Main.Client is used to control the service: start or stop it and also receive
  callbacks when service encounters an error, is killed, is started or is
  stopped.
Makefile.am: change android apk name

If no name is specified, it'll use the name of the first activity of the
AndroidManifest.xml. And we don't want the apk to be called Settings.apk.
android: run Main service in a separate process

If MPD crashes, Settings Activity won't be killed and will be notified that a
crash occurred.