java7: Implement Character.UnicodeScript

Implemented in terms of u_getPropertyValueEnum(UCHAR_SCRIPT,)
and uscript_getScript from ICU4C.

Since the Java 7 API specifies a subset of scripts known to
ICU4C, we map scripts not specified in the API to
UnicodeScript.UNKNOWN.

Like the UnicodeBlock API, our matching is far more lenient than
the RI.

Change-Id: I5247baddb2dde42d6a2042609370daeb616fe419