Android: Add githash header to spare rebuilds after new commits

Before, android_version.h got changed at every new commit. Now, we
only change it with new minetest releases. Analogous to how cmake
does it,  we add an android_version_githash.h file that communicates
the git hash to C++ code.

Also, unify VERS_MAJOR, VERS_MINOR and VERS_PATCH variable
calculation inside the whole makefile.