Android SSO (Single sign-on) for app

Question

Is there any free single-signon mechanism for Android that gracefully works with non-webapps? Say, something along the lines of...

1. You launch my app and use

Answer 1

What you described in Alternatively section looks like Authorization Code using Proof Key for Code Exchange (PKCE) OAuth 2.0 grant

https://tools.ietf.org/html/rfc7636

The flow is the following:
1. Your app generates a random key (code_verifier) and it's transformed value (code_challenge) and navigates to authorization server with code_challenge as a parameter
2. Authorization server redirects back to your app with authorization code
3. Your app sends authorization_code and the code_verifier to the auth server to get the authorization token
4. Server uses authorization_code and the code_verifier (it already has code_challenge, so it can verify that code_verifier is correct) and then returns to your app with the token.

On the practical side of it you can use something like Auth0 to implement it in your app.