Well all works till i generate the signed apk . I followed the entire process as told on the google developers page
1.I generated the google-services.json file with
It is hardly difficult to find the place to activate inside the huge developer console. May be I will help somebody in the future. You have to register your Android application here - "enable Sign-In" in this page:
Just input usual for console developer values this your signed SHA-1. That's it!