Improve presubmit repo hook to check license headers

This CL adds a presubmit check for files under {xml,json}/src/main to ensure
they have no GPL license; and narrows luni/ and ojluni/ to luni/src/main
and ojluni/src/main for some speed optimization.

Renamed checkstyle-luni check to checkstyle-luni-json-xml to have these
directories checked during single checkstyle run to save some time.

Bug: 38121042
Test: Added forbidden licenses to luni/ and ojluni/, run pre-upload.py and got the expected failure.
Test: Added forbidden licenses to json/ and xml/, run pre-upload.py and got the expected failure.
Change-Id: Ibeea9cce6634ac59a4ba1d334ad031515ab862ce
1 file changed