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/res/layout/custom_notification_gb.xml

@@ -0,0 +1,22 @@
+<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
+    android:id="@+id/layout"
+    android:layout_width="fill_parent"
+    android:layout_height="fill_parent"
+    android:padding="10dp" >
+    <ImageView android:id="@+id/image"
+        android:layout_width="wrap_content"
+        android:layout_height="fill_parent"
+        android:layout_alignParentLeft="true"
+        android:layout_marginRight="10dp" />
+    <TextView android:id="@+id/title"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:layout_toRightOf="@id/image"
+        style="Custom Notification Title" />
+    <TextView android:id="@+id/text"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:layout_toRightOf="@id/image"
+        android:layout_below="@id/title"
+        style="Custom Notification Text" />
+</RelativeLayout>

android/res/values/strings.xml

@@ -2,4 +2,6 @@
 
 <resources>
   <string name="app_name">MPD</string>
+  <string name="notification_title_mpd_running">Music Player Daemon is running</string>
+  <string name="notification_text_mpd_running">Touch for MPD options.</string>
 </resources>