Merge: Android: extract input_events and clean up

Extract `input_events` from `android_app` so it is independant of the display part of Mnit. It now relies only on `mnit_input`, which will probably be kept around for Gamnit.

The extraction has been organized in three commits for easier review:
* Extract the code with minimal modifications.
* Update the style of the new module (this may be the first module that used the FFI, its style was outdated).
* Improve the doc and fix warnings in the new module.

Pull-Request: #1075
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Romain Chanoir <chanoir.romain@courrier.uqam.ca>