java7: Implement new Locale APIs.

Most validation and sanity checking is performed entirely
in java. We use ICU only for converting back and forth
between Locale objects and BCP-47 language tags
(toLanguageTag & forLanguageTag).

The new APIs also add support for script subtags and
manipulation of unicode locale extensions.

Note that we do not implement
IllformedLocaleException.getErrorIndex. If anyone complains
or if it proves useful somehow, we could plumb the information
through from ICU if required.

bug: 8992787
Change-Id: I9b2d7f8d17f970d1b57111dcfe92c16e68b0fa67