Allow running make_apk.py from any directory

per XWALK-2391, prepends the current location of make_apk.py when accessing files and directories assumed to be in the same directory as make_apk.py, including imports of other modules.
> Thanks, I don’t plan to be an owner, actually. In the contributing
> guidelines
> (https://crosswalk-project.org/#contribute/contributing-code) it said:
>  “You should also add your name to the AUTHORS file the first time you make a patch.”
> I didn’t find the AUTHORS file and thought OWNERS was used instead.
> Please disregard it.

Nice catch; the AUTHORS file was removed several months ago when we
formalized our decision not to require a CLA, but apparently the website
was not updated.

I've filed
https://github.com/crosswalk-project/crosswalk-website/pull/152 to fix
this.