Disable compiler's unused constants check for libicui18n

Since ICU 65.1 there are three unused constants in libicui18n headers
which libandroidicu depends on. This CL disables compiler's check for
such unused constants to prevent a build failure caused by enabled -Wall
-Werror options in libicui18n.

This issue is an upstream bug which is tracked by http://b/146546882 and
will be fixed in a separate CL.

Bug: 146546882
Test: m libicui18n
Change-Id: Ie02691b1f4e5beeaf2016fef224a7b1b736fe62e
1 file changed