I'd like to implement an OAUTH2 authorization flow natively in Android without a webview. I have found many tutorials that show how to use OAUTH2 using a webview, but haven't found any that perform the redirects and login behind the scenes using JSON, login/pw fields, and native android components (minus the webview) Has anyone done this or have any suggestions on how to do this?
I suggest you look at App-Auth and review this IETF Draft OAuth 2.0 for Native Apps
The Gluu Server will support PKCE in version 2.4.4 (due out the end of April) if you're looking to deploy an OpenID Connect Provider that supports this flow. Check out the docs for more info on how to deploy.