Android Example Solution for: Google Sign In Error 12500 x Internal Testing

The problem: If anyone is using the Google Play's Internal Testing feature to make your app available for selected users, be aware that Google changes the app and therefore the APK signature - so Firebase-based services that require SHA-1 signature will NOT work properly.

In my case, Google Sign In was returning error 12500. Google's documentation were not very helpful to find out why:
public static final int SIGN_IN_FAILED
The sign in attempt didn't succeed with the current account.

Unlike SIGN_IN_REQUIRED, when seeing this error code, there is nothing user can do
to recover from the sign in failure. Switching to another account may or may not help.
Check adb log to see details if any.

Constant Value: 12500

The solution: You can either avoid Internal Testing feature or temporarily adding the derived APK SHA-1 signature by downloading it from Google Play (derived version) and then extracting SHA-1 signature using Java's keytool. Happy coding :)