Few (but some) RN modules rely on the main Application implementing
ReactApplication.

For Detox to actually work on Android we need the app to be a greenfield
app. Specifically, the main Application must implement ReactApplication.

In order to satisfy this requirement we are introducing a helper
ReactNativeHost implementation which encapsulates our RN integration,
which is what the ReactApplication needs to expose.

While we don't really need this ourselves (except if we end up adopting
Detox) we have known people who fork our app and add dependencies that have
this requirement to it, so this change will help them too.