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.
Please note that you're not supposed to add yourself to `OWNERS` like that -- [the process]( https://crosswalk-project.org/#contribute/reviewer_policy) is similar to Chromium's and involves an existing owner proposing your addition after a certain amount of significant contributions has been made.