Use ICU in libandroidicu

libicuuc and libicui18n are moved into APEX, but
they have no stable ABI due to the version suffix.

Use libandroidicu which provides stable symbol.
See http://go/apex-stable-icu4c-interface for the design.

libxml2 starts depending on libandroidicu.
On the host side, same dependency is also needed.
Note that any host binary can still depend on libicuuc directly,
since the APEX is not updatable on the host side.

Bug: 117094880
Test: m checkbuild
Change-Id: I52ee3f2600922931ceb1b55b1558135a596c69b8