Enhancement: Faster browser authentication

Login flow has been improved by saving a click when the server is OAuth2/OIDC and it is valid. Also,
when authenticating again in a OAuth2/OIDC account already saved in the app, the username is already shown
in the browser.

https://github.com/owncloud/android/pull/3632
https://github.com/owncloud/android/issues/3759
