i have a button and i know a user's unique UID generated by firebase after a user is created on the firebase console's authentication page, now upon clicking the button i want to login that user, is that possible? if yes how?
i don't want to use email and password, i want to login with just the UID.

I have tried creating custom token as shown here [Cannot Resolve setCredentials in FirebaseOptions.Builder().setCredentials(...)](https://stackoverflow.com/questions/53207292/cannot-resolve-setcredentials-in-firebaseoptions-builder-setcredentials)

In my application i'm using a face recognition api, so the user can login by two ways:
1)manually by entering email, password and clicking sign in.
2)in the sign in page there is a button to sign in with face, clicking on which it scans the face and returns the UID of the user if exists, now i want to signin that user. 

please help.