Migrate AeroGearGCMPushRegistrar to FCM

The 'AeroGearGCMPushRegistrar' class needs FCM changes

1) rename to reflect the FCM usage
2) the 'GcmPubSub' functionality is now part of 'FirebaseMessaging', and does have more fluent APIs to (un)subscribe from/to a topic.

Changes needed in here:
https://github.com/aerogear/aerogear-android-push/blob/3.0.1/aerogear-android-push/src/main/java/org/jboss/aerogear/android/unifiedpush/gcm/AeroGearGCMPushRegistrar.java#L176-L183

For instance:

...
FirebaseMessaging.getInstance().subscribeToTopic("mytopic");
...

More details are here: https://developers.google.com/cloud-messaging/android/android-migrate-fcm#update_the_usage_of_gcmpubsub