Respond to feedback from Ity re: AaptGen

- Extract package_path upwards to be a common method
- create an IGNORED_ASSETS constant so as to move the
  definition out of the method scope
- block comment explaining used aapt flags
- properly ordered @classmethods
- Various style fixes, including consistency in
  naming private vars

Since I was here doing work, I closed an existing TODO
which called for "ignored-assets" to be configurable.
It has been tested to work.