Merge: Android: select the low-level implementations from other importations

This PR needs some context first: there is two low-level implementations for Nit apps on Android. The oldest is based on C code and `native_app_glue`, it is used by all our game engines. The new one is based on a custom Java class `NitActivity`, it is used to implement `app::ui` and should replace the oldest one when I have time to work on it.

Prior to this PR, importing `android` imported the old C implementation, and you had to import `android::ui` to get the Java implementation.

This PR changes this behavior and uses conditional importations to choose between the two low-level implementations. So importing `android` and `gamnit` uses the old C implementation (now `android::game`) and importing `android` and `app::ui` uses the new Java implementation (`android::nit_activity`).

This allows us to use the simple `nitc src/calculator.nit -m android`, instead of `nitc src/calculator.nit -m ../../lib/android/ui.nit`. And the most important is that the user doesn't have to chose between (or understand) the two low-level implementations.

Pull-Request: #2093
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Romain Chanoir <romain.chanoir@viacesi.fr>