Demo apps in Kotlin

### [REQUIRED] Use case description
ExoPlayer's demo apps should be written in Kotlin as it becomes the preferred language for Android development.

In addition to our full demo app, we should also add more targeted demos to better demonstrate certain features without bloating the demo app with everything at once.

Ideas:
- super simple hello world example
- audio only playback (including backgrounding)
- You can look at [UAMP](https://github.com/android/uamp) which is written in kotlin, includes a background service, and cast integration.
- playlists
- downloads
- ad playback [exists already]
- cast integration [exists already]