- https://productforums.google.com/forum/#!msg/nexus/fslYqYrULto/lU2D3Qe1mugJ
- http://stackoverflow.com/questions/27471141/gcm-how-to-send-heartbeat-to-gcm-server
- http://stackoverflow.com/questions/19560448/how-to-avoid-delay-in-android-gcm-messages-change-heartbeat
- https://stackoverflow.com/questions/49834694/all-firebase-libraries-must-be-either-above-or-below-14-0-0
- https://android-developers.googleblog.com/2018/05/announcing-new-sdk-versioning.html?m=1
- https://developers.google.com/android/guides/releases
- https://firebase.google.com/support/release-notes/android