Merge: calculator: nicer Android adaptation with API 21 + variant

Improve the look of the Android variant for the calculator using style resources from the AOSP calculator. Also intro another variant for Android API 21+ which changes the font and deactivates the virtual keyboard.

This should also fix the calculator missing from our f-droid repository. However, I don't know yet how f-droid handles more than one APK per package...

For an idea of the result, you can take a look at the 6 different resulting calculators alongside the latest AOSP calculator: http://xymus.net/pub/calcs.pdf

Pull-Request: #2255
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Ait younes Mehdi Adel <overpex@gmail.com>