Add an Android app

A long-term goal would be to have an Android app too that integrates the `DistributedChat` package with a platform-specific UI. There are a few challenges with this approach, however:

* Building the Swift compiler for Android is possible, but non-trivial ([official docs are here](https://github.com/apple/swift/blob/main/docs/Android.md))
* Integrating the `ChatController` may require wrapping it into C-compatible structures for interfacing with the JVM world (Java, Kotlin, ...)
* This requires using the Android NDK and JNI

A UI could be written using a high-level framework for Kotlin, e.g. [Jetpack Compose](https://developer.android.com/jetpack/compose) which is similar to SwiftUI.