android: add super classes to `NativeActivity`

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/android: intro the `ui` module

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
examples: intro app.nit Android UI test

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
calculator: add an icon

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
calculator: intro android UI

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>