ICU-20140 Allow duplicated keys in U-extension per RFC 6067 (#136)

ICU-20140 Allow duplicated keys in U-extension per RFC 6067 (#136)

* ICU-20140 Allow duplicated keys in U-extension per RFC 6067

RFC 6067 [1] does allow duplicate keywords, but ICU4C's
uloc_forLanguageCode rejects it as invalid.

Change it to accept duplicate keywords and honor only the
1st one while ignoring subsequent ones per RFC 6067.

[1] Unicode extension to BCP 47:
    https://tools.ietf.org/html/rfc6067

* ICU-20140 Add ICU4J test and tweak ICU4C test

ICU4J test diverges from ICU4C tests:

1. Handling of duplicate variants in ICU4J seem to be wrong:
   https://unicode-org.atlassian.net/browse/ICU-20148
2. ULocale.forLanguageTag only throws NullPointException so
   that ICU4C's test for duplicate attributes cannot be ported.