This includes the following changes:
- move this developer-only script to test/ where it conceptually belongs
- this also prevents it from being distributed with this package
- fix paths for `android` and `androidx` variants
- make paths relative to the script, not to CWD
- use `removeSync` instead of `existsSync` and `existsSync`
- rename npm script to `clean:java-unit-tests` to clarify scope
