Make GCM endpoint swappable and default to new FCM endpoint

[bbangert]

The GCM endpoints will be shutdown April 2019. GCM client tokens however will apparently not be:
https://developers.google.com/cloud-messaging/faq

To send to GCM clients, we can continue to send identically, but to the new FCM endpoints, per:
https://developers.google.com/cloud-messaging/android/android-migrate-fcm#update-server-endpoints

We must have this tested and deployed before April to avoid delivery issues to Android clients still using GCM tokens/SDK.

[bbangert]

We might also have to contact whoever owns the Google Cloud Project for the apps using GCM, to import the GCM project as a Firebase project per:
https://developers.google.com/cloud-messaging/android/android-migrate-fcm#import-your-gcm-project-as-a-firebase-project