android: get version code from date and time

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
android: use version code in manifest

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
Merge: Android version code

This should help FDroid understand our packages.

Pull-Request: #433
Reviewed-by: Jean Privat <jean@pryen.org>