Update/revise version code

This MR implements a brand-new version code scheme for Ceno! Instead of depending on some strange python script copied from old versions of firefox, we are now crafting a sensible version code from the version number, target architecture, revision number. It has the added bonus of allowing us distinguish fdroid builds of the apk. See the related issue #124 (closed) for more info on the format of this.

Please review thoroughly, because once we go with this scheme, there is no turning back (or at least no possibility of using version codes lower than 2020010000).