WIP ICU-12968 remove icudata.jar from icu4j tree

[srl295]

• improve build of data
• :construction: does NOT work on windows
• add a convenience script on the icu4j side
• probably some other holes here

Checklist

• [x] Issue filed at https://unicode-org.atlassian.net/browse/ICU-12968
• [x] Update PR title to include Issue number
• [ ] Issue accepted (kind of)
• [ ] Documentation is changed or added (kind of)

[srl295]

https://travis-ci.org/unicode-org/icu/jobs/431690519#L5651 successfully built ICU4J with no .jar files in the repository… (added 4 minutes to build time)

[srl295]

https://github.com/unicode-org/icu/commit/b8c4e91dc7e7130bd9282bbd178bc0ed6af41fdf#diff-d32048782dc30c3615e42837d446d6b4 is an alternate approach ( and eerily similar, written a year and a half earlier which I obviously forgot about ) where icu4j calls icu4c to build.

[markusicu]

@srl295 are you intending to push this further?

[srl295]

@markusicu needs a port to Windows and other cleanup …I'm not really able to do much with this. It's still a good idea. Maybe something to put under consideration with any build system redo…