It help to fallback to UnifiedPush (if available) if the PlayServices are not installed on the device.
Firebase
Configuration
In order to make this module only know about Firebase, the plugin com.google.gms.google-services has been disabled from the app module.
To be able to change the values in the file firebase.xml from this module, you should enable the plugin com.google.gms.google-services again, copy the file google-services.json to the folder /app/src/main, build the project, and check the generated file app/build/generated/res/google-services/<buildtype>/values/values.xml to import the generated values into the firebase.xml files.