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

target/product/default_art_config.mk

@@ -18,16 +18,37 @@ ifeq ($(ART_APEX_JARS),)
   $(error ART_APEX_JARS is empty; cannot initialize PRODUCT_BOOT_JARS variable)
 endif
 
-# The order matters for runtime class lookup performance.
+# Order of the jars on BOOTCLASSPATH follows:
+# 1. ART APEX jars
+# 2. System jars
+# 3. System_ext jars
+# 4. Non-updatable APEX jars
+# 5. Updatable APEX jars
+#
+# ART APEX jars (1) are defined in ART_APEX_JARS. System, system_ext, and non updatable boot jars
+# are defined below in PRODUCT_BOOT_JARS. All updatable APEX boot jars are part of
+# PRODUCT_UPDATABLE_BOOT_JARS.
+#
+# The actual runtime ordering matching above is determined by derive_classpath service at runtime.
+# See packages/modules/SdkExtensions/README.md for more details.
+
+# The order of PRODUCT_BOOT_JARS matters for runtime class lookup performance.
 PRODUCT_BOOT_JARS := \
-    $(ART_APEX_JARS) \
+    $(ART_APEX_JARS)
+
+# /system and /system_ext boot jars.
+PRODUCT_BOOT_JARS += \
     framework-minus-apex \
     ext \
-    com.android.i18n:core-icu4j \
     telephony-common \
     voip-common \
     ims-common
 
+# Non-updatable APEX jars. Keep the list sorted.
+PRODUCT_BOOT_JARS += \
+    com.android.i18n:core-icu4j
+
+# Updatable APEX jars. Keep the list sorted.
 PRODUCT_UPDATABLE_BOOT_JARS := \
     com.android.conscrypt:conscrypt \
     com.android.media:updatable-media \