Google OAuth authenticate as page account

Question

I'm lost on this one. The problem is simple: what is the correct, recommended way in Android of logging in a Google user, as a page that they own (e.g. they first login and they are presented with what page they wish to authenticate as, to the app)?

The web solution is straightforward - Google's /oauth endpoint automatically redirects to a delegation page into which you choose your main account or the page you wish to continue as. The next page will have a pageId=... extra argument in the URL. As such, your authorization is linked to that specific page, not to the main account.

With Android, I'm completely lost. Every possible official guide out there is either deprecated, un-recommended, or just simply, blantly ignores this simple use-case and only works with getting an OAuth token for the main account, without even a word given to help for the case of users that own pages which can be used as their own identity under the user's account.

What I'm doing now (at the high level), using Google Play Services:

1. Start an activity intent via AccountPicker.newChooseAccountIntent
2. User selects an account on device.
3. Calling GoogleAuthUtil.getToken to get an OAuth token - this triggers the basic OAuth flow which asks the user to authorize the app - NO ACCOUNT SELECTION at this step, like the web flow does!

What I looked at:

1. Google HTTP Api Client - this one recommends to use Android's AccountManager and never use a "custom" solution to save any refresh tokens, etc. Completely useless for the use-case.
2. Google Sign-In - this one seems to require a server end-point (wtf?). I tried using it and adding in the OAuth scopes I'm interested in, it didn't even work. I'm not interested in the user's email, and this API seems to be oriented for signing users up rather than authenticating as a Google one.

Other apps I looked at seem to just open a WebView and force the user to login in that, which triggers the normal web flow, with account/page selection after login. I really don't like this since it requires a separate login.

Is there an official stand on this issue and I'm not seeing it?