Move core-icu4j to PRODUCT_UPDATABLE_BOOT_JARS.

... but not really: move it back to PRODUCT_BOOT_JARS before the list is
passed to soong. This ensures that core-icu4j remains on the boot image
for system health reasons and maintains the status quo.

core-icu4j is the only APEX jar (aside from ART jars) that is ever meant
to be part of the boot image. This is an implementation detail that most
devs don't and shouldn't care about. Instead, most devs assume that
PRODUCT_UPDATABLE_BOOT_JARS is where all apex jars are meant to go. In
the follow ups, PRODUCT_UPDATABLE_BOOT_JARS would be renamed to
PRODUCT_APEX_BOOT_JARS, regardless of the updatable property.

This also solves the problem where vendors add /system and /system_ext
boot jars to PRODUCT_BOOT_JARS outside of default_art_config.mk and
essentially violate the ordering requirement: all /apex jars come after
/system and /system_ext.

Bug: 191127295
Test: atest CtsClasspathsTestCases derive_classpath_test
Change-Id: Ifdfdd02519a0f5baea45619523f0c1eb8be186bc