Merge: Reorganize mnit submodules

So now Linux clients import `mnit::linux` instead of `mnit_linux` and Android clients import `mnit::android`. Also rename `mnit_display` and `mnit_input` to `mnit::display` and `mnit::input` respectively.

This may look familiar as it was a part of #1671.

Pull-Request: #1674
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>