Git threw away my changes to a file that had been renamed.

Manually patch the ICU.cpp changes into libcore_icu_ICU.cpp. Also remove the
whole sentence from the class documentation, since it looks rather odd without
the bit that would refer to @hidden API.

Bug: 3484927
Change-Id: I1ea046e18949ad5bfa7f4512df1f481c81a94310