Using the recommended backwards compatible way of scheduling jobs introduced in Android O.

We moved from using `IntentService` to `JobIntentService` in preparation for this change but the call to schedule jobs remained the same which meant that our custom IntentService was not doing any work at all.

Using `JobIntentService.enqueueWork`, when running on Android O or later, the work will be dispatched as a job via JobScheduler.enqueue. When running on older versions of the platform, it will use Context.startService.
https://developer.android.com/reference/androidx/core/app/JobIntentService

Fixes
firebase/quickstart-unity#877
firebase/quickstart-unity#883

PiperOrigin-RevId: 350176964

Author: Googler

messaging/src/android/java/com/google/firebase/messaging/JobIds.java

@@ -0,0 +1,23 @@
+// Copyright 2020 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.firebase.messaging;
+
+/** Unique ids for JobIntentServices. */
+public final class JobIds {
+
+  private JobIds() {}
+
+  public static final int MESSAGE_FORWARDING_SERVICE = 1000;
+}

messaging/src/android/java/com/google/firebase/messaging/MessageForwardingService.java

@@ -16,10 +16,12 @@
 
 import android.content.Context;
 import android.content.Intent;
+import android.os.Bundle;
 import androidx.core.app.JobIntentService;
 import com.google.firebase.messaging.cpp.DebugLogging;
 import com.google.firebase.messaging.cpp.MessageWriter;
 
+
 /**
  * Listens for Message intents from the application and sends them to the C++ app via the
  * ListenerService.
@@ -28,6 +30,14 @@ public class MessageForwardingService extends JobIntentService {
   private static final String TAG = "FIREBASE_MSG_FWDR";
   public static final String ACTION_REMOTE_INTENT = "com.google.android.c2dm.intent.RECEIVE";
 
+  /**
+  * Convenience wrapper over enqueueWork to either directly start the service (when running on
+  * pre-O platforms) or enqueue work for it as a job (when running on Android O and later).
+  */
+  public static void enqueueWork(Context context, Intent intent) {
+    enqueueWork(context, MessageForwardingService.class, JobIds.MESSAGE_FORWARDING_SERVICE, intent);
+  }
+
   // Handle message intents sent from the ListenerService.
   @Override
   protected void onHandleWork(Intent intent) {
@@ -46,7 +56,7 @@ static void handleIntent(Context context, Intent intent, MessageWriter messageWr
     if (intent != null
         && intent.getAction() != null
         && intent.getAction().equals(ACTION_REMOTE_INTENT)) {
-      android.os.Bundle extras = intent.getExtras();
+      Bundle extras = intent.getExtras();
       DebugLogging.log(TAG, "extras: " + (extras == null ? "(null)" : extras.toString()));
       if (extras != null) {
         RemoteMessage message = new RemoteMessage(extras);