Include icu4c libraries to the list of public libs

Note that with this change the linker will ignore
icu libraries provided by an app because public
libraries are always provided by system. At this point
it does not affect any apps because before Marshmallow
all of them were getting platform version of icu4c
libs anyways (they will be getting their own copy of the
library only in Marshmallow release).

Bug: http://b/26217329
Bug: http://b/26929349
Change-Id: I681cab43c9f28ec22319d9f0424bd3df00743a5f