Do not generate cargo_pkg_version when --no-pkg-vers is passed

If you passed --no-pkg-vers to cargo2android.py, it would still
generate cargo_pkg_version with an empty string.  Instead, simply
don't generate anything.

Test: Use with and without argument.
Change-Id: I544e4789312ccc3481da73c78f0f2cae2ad6619c
1 file changed