Kotlin: adding Cognito Flutter mobile app

[AMZ-brandon]

This pull request adds an Amazon Cognito example app for Android in Flutter and Kotlin. This app is part of a documentation update for the Cognito developer guide. @bhatti is primary contributor.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[scmacdon]

Only comment I have is to move this larger example to kotlin/usecases.

[AMZ-brandon]

Only comment I have is to move this larger example to kotlin/usecases.

@scmacdon Done!

[AMZ-brandon]

@scmacdon do you have any recommendations to get past this error?
/github/workspace/kotlin/usecases/cognito_flutter_mobile_app/android/app/src/main/kotlin/com/example/android-app/MainActivity.kt:4:28: Not a valid Kotlin file (4:28 expecting a top level declaration) (cannot be auto-corrected) ()

The app works, so I'm hoping there's an easy way to remedy this. Also @bhatti for any ideas.
File contents:

// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
// SPDX-License-Identifier: Apache-2.0

package com.example.android-app

import io.flutter.embedding.android.FlutterActivity

class MainActivity : FlutterActivity() { /*...*/ }

[bhatti]

@AMZ-brandon , I am not quite following the cause of kotlin error as that file was generated by flutter tooling and is a valid kotlin file. The developers of flutter generally use dart for the application logic and flutter tooling generates scaffolding for target platform. As, flutter project includes multiple languages including kotlin, dart, objective-c, c++, etc so it probably needs its own folder under dart. Let me know what you think.