Added partial Android FCM support (#975)

* Modified plugin.xml to include FCM changes

* Plugin.xml changes for FCM

* Java changes for FCM

* Increased plugin version to 2.0.0

* Increased plugin version in package.json

* Added topic subscription/unsubscription

* Removed some empty lines

plugin.xml

@@ -1,5 +1,11 @@
-<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
-<plugin xmlns="http://www.phonegap.com/ns/plugins/1.0" xmlns:android="http://schemas.android.com/apk/res/android" xmlns:amazon="http://schemas.android.com/apk/lib/com.amazon.device.ads" xmlns:rim="http://www.blackberry.com/ns/widgets" id="phonegap-plugin-push" version="2.0.0-rc2">
+<?xml version="1.0" encoding="UTF-8"?>
+<plugin xmlns="http://www.phonegap.com/ns/plugins/1.0"
+    xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:amazon="http://schemas.android.com/apk/lib/com.amazon.device.ads"
+    xmlns:rim="http://www.blackberry.com/ns/widgets"
+    id="phonegap-plugin-push"
+    version="2.0.0">
+
   <name>PushPlugin</name>
   <description>
     This plugin allows your application to receive push notifications on Android, iOS and Windows devices.
@@ -96,4 +102,4 @@
       <preference name="WindowsToastCapable" value="true"/>
     </config-file>
   </platform>
-</plugin>
+</plugin>

src/android/com/adobe/phonegap/push/PushPlugin.java

@@ -346,7 +346,10 @@ private void subscribeToTopics(JSONArray topics, String registrationToken) {
             String topic = null;
             for (int i=0; i<topics.length(); i++) {
                 topic = topics.optString(i, null);
-                subscribeToTopic(topic, registrationToken);
+                if (topic != null) {
+                    Log.d(LOG_TAG, "Subscribing to topic: " + topic);
+                    FirebaseMessaging.getInstance().subscribeToTopic(topic);
+                }
             }
         }
     }