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/IMain.aidl

@@ -0,0 +1,12 @@
+package org.musicpd;
+import org.musicpd.IMainCallback;
+
+interface IMain
+{
+    void start();
+    void stop();
+    void setWakelockEnabled(boolean enabled);
+    boolean isRunning();
+    void registerCallback(IMainCallback cb);
+    void unregisterCallback(IMainCallback cb);
+}