InternetManager refactoring

Refactor `InternetManager`:

1. Add an interface. Change the name to `InternetConnectivityManager`.
2. Use `callbackFlow` to get a cold `Flow` of connectivity states, get rid of the `SharedFlow`.
3. Think about connectivity states. How many do we actually need? Maybe a simple boolean is enough.

Motivation:
https://www.youtube.com/watch?v=TzV0oCRDNfM