Move core-icu4j.jar down in PRODUCT_BOOT_JARS.

For go/updatable-bootclasspath it would simplify the logic if all
system boot jars were in a single block, instead of having some apex
jars in between them.

core-icu4j.jar used to be part of ART_APEX_JARS before it moved to its
own apex. However, this change puts it after system jars in relative
ordering.

Bug: 180105615
Test: presubmit
Change-Id: Icadc1b67191172bb02d1a15bdfa3d2e6f69227aa