Update public documentation for C++ FCM SDK. (#789)

As of Firebase C++ SDK 7.1.0, we started using JobIntentService for scheduling jobs. This change requires a few additional modifications to AndroidManifest.xml and a new way of starting a background service.

These changes were done to the old repository, and need to be brought to GitHub.

Author: a-maurice

messaging/samples/AndroidManifest.xml

@@ -62,6 +62,13 @@
           </intent-filter>
         </service>
         <!-- [END messaging_add_custom_listener_service] -->
+
+        <!-- [START messaging_forwarding_service] -->
+        <service android:name="com.google.firebase.messaging.MessageForwardingService"
+             android:permission="android.permission.BIND_JOB_SERVICE"
+             android:exported="false" >
+        </service>
+        <!-- [END messaging_forwarding_service] -->
     </application>
 
     <uses-permission android:name="android.permission.INTERNET"/>

messaging/samples/src/java/com/google/firebase/messaging/cpp/samples/MyActivity.java

@@ -36,7 +36,9 @@ protected void onNewIntent(Intent intent) {
     message.setAction(MessageForwardingService.ACTION_REMOTE_INTENT);
     message.putExtras(intent);
     message.setData(intent.getData());
-    startService(message);
+    // For older versions of Firebase C++ SDK (< 7.1.0), use `startService`.
+    // startService(message);
+    MessageForwardingService.enqueueWork(this, message);
   }
 }
 // [END messaging_on_new_intent]