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/AndroidManifest.xml

@@ -8,14 +8,14 @@
   <uses-sdk android:minSdkVersion="21" android:targetSdkVersion="26"/>
 
   <application android:icon="@drawable/icon" android:label="@string/app_name">
-    <activity android:name=".Main"
-              android:label="@string/app_name"
-              android:launchMode="singleInstance">
+    <activity android:name=".Settings"
+              android:label="@string/app_name">
       <intent-filter>
         <action android:name="android.intent.action.MAIN" />
         <category android:name="android.intent.category.LAUNCHER" />
       </intent-filter>
     </activity>
+    <service android:name=".Main" />
   </application>
 
   <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>