The Confluence Server iOS/Android apps use the WebView UI element on Android and UIWebView on iOS to render the login page for the app. This causes a 403: disallowed_useragent error when attempting to login with Google OAuth due to the changes outlined here: https://developers.googleblog.com/2016/08/modernizing-oauth-interactions-in-native-apps.html. 

An alternative would be to use the device browser for rendering the login view instead of the web-views.