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.

- Main.start to start the service without any fallback.

android/src/IMainCallback.aidl

@@ -0,0 +1,9 @@
+package org.musicpd;
+
+interface IMainCallback
+{
+    void onStarted();
+    void onStopped();
+    void onError(String error);
+    void onLog(int priority, String msg);
+}