Merge: Intro NitActivity an Android entry point in pure Nit, Java, and C (almost...

This PR could be described in 3 steps.

* Clean up lib/android and move up `dalvik` in the module hierarchy.
* Intro NitActivity.java (the most important commit)
* Update calculator (only) to use the new NitActivity with all its perks.

See the doc of the nit_activity module for the details on the polyglot implementation, copied hre for your convenience:

This module is implemented in 3 languages:

* The Java code, in `NitActivity.java` acts as the entry point registered
   to the Android OS. It relays most of the Android callbacks to C.
   In theory, there may be more than one instance of `NitActivity` alive at
   a given time. They hold a reference to the corresponding Nit `Activity`
   in the attribute `nitActivity`.

* The C code is defined in the top part of this source file. It acts as a
   glue between Java and Nit by relaying calls between both languages.
   It keeps a global variables reference to the Java VM and the Nit `App`.

* The Nit code defines the `Activity` class with the callbacks from Android.
   The callback methods should be redefined by user modules.

Pull-Request: #1208
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>