Simplify usage of signapk.py on CI

To simplify the usage of the signapk.py script on CI, I have extended signapk.py, so that it checks whether the current project branch is cleared for signing. If the branch isn't cleared, then the script does nothing, but exits with success, so that a job calling this script doesn't fail. See ci-utilities!139 (merged) for how this looks in our Android jobs.

The first three commits are just a simple renaming of a class and a bit of refactoring to help the static type checker mypy infer correct types.