Bug 1252650 - Support loading PushService immediately on Android; r=kitcambridge

Normally we delay-load PushService on session restore. However, we won't
have session restore when Gecko is running in background without GUI. So
we need a way to load PushService immediately.

dom/push/Push.manifest

@@ -6,3 +6,6 @@ contract @mozilla.org/push/PushManager;1 {cde1d019-fad8-4044-b141-65fb4fb7a245}
 component {daaa8d73-677e-4233-8acd-2c404bd01658} PushComponents.js
 contract @mozilla.org/push/Service;1 {daaa8d73-677e-4233-8acd-2c404bd01658}
 category app-startup PushServiceParent @mozilla.org/push/Service;1
+
+# For immediate loading of PushService instead of delayed loading.
+category android-push-service PushServiceParent @mozilla.org/push/Service;1

dom/push/PushComponents.js

@@ -83,6 +83,11 @@ PushServiceBase.prototype = {
       this._handleReady();
       return;
     }
+    if (topic === "android-push-service") {
+      // Load PushService immediately.
+      this._handleReady();
+      return;
+    }
   },
 
   _deliverSubscription(request, props) {