Merge pull request #3 from alessandro40/master

Add the Java code for the prototype Android app
res: remove now-unneeded .gitkeep
src: remove now-unneeded .gitkeep
MainActivity.java: mark loadLibrary with XXX

IIUC, onCreate() is called multiple times during
the Android App lifetime.

The library, on the contrary, should probably
only be loaded once.

Hence, the XXX comment.