Added auth demp app

[lalwani]

• Created a demo app using authentication module
• app supports prefilling user information
• demonstrates how to use app priority
• shows a toast message for token or error

[RunTerror]

I'm also trying to perform cross platform sso but on hitting this function, it opens a blank white screen and then comes back to the page again.

 val authContext = AuthContext(
               authDestination = AuthDestination.InApp ,
                authType = AuthType.AuthCode,
                prefillInfo = prefillInfo ?: prefill,
                prompt = Prompt.LOGIN,
            )


        UberAuthClientImpl().authenticate(this, launcher, authContext)