android: Add intents for service start and stop

org.musicpd.action.StartService
org.musicpd.action.StopService

You can test these actions like:
adb shell am broadcast -a org.musicpd.action.StartService org.musicpd

Calling these from an app like tasker should allow for automation

android/app/src/main/AndroidManifest.xml

@@ -53,6 +53,16 @@
             </intent-filter>
         </receiver>
 
+        <receiver android:name=".AutomationReceiver"
+            android:exported="true">
+            <intent-filter>
+                <action android:name="org.musicpd.action.StartService" />
+            </intent-filter>
+            <intent-filter>
+                <action android:name="org.musicpd.action.StopService" />
+            </intent-filter>
+        </receiver>
+
         <service
             android:name=".Main" />
     </application>