Stop crashlytics poisoning by developer builds

[davecraig]

The aim here is to stop developers who have a valid google-services.json file from automatically reporting development crashes against release versions. It's not clear the best way to do this. The current approach is that there is a dummy google-services.json file which doesn't point to our app. However, this results in lots of errors in the logs which make them harder to read. Ideally we'd have a valid JSON file that either pointed somewhere other than our app, or had a different it so that crashes by developers weren't counted against release versions. There must be a standard way of doing this.