Cloud Endpoints + Firebase Admin SDK: authenticating users via their FirebaseUser tokens

I have build a mobile backend for my android application using google cloud endpoints (version 1, using android studio). I wish to authenticate my users via email/password, so I am using Firebase Authentication to do so. The Firebase Authentication sdk allows me to get each user's token on the client side (in android), and the firebase admin sdk allows me to check the validity of a token on the backend. I understand that in cloud endpoints I can provide my own custom authenticator (see: I have build a mobile backend for my android ap