Use two-column format for PRODUCT_BOOT_JARS components.

The first component is the apex name, or a special name "platform"
if the boot jar is a platform jar rather than a part of some apex.
This is a prerequisite change for moving core-icu4j to a separate
com.android.i18n apex.

Old one-column format is still supported, but all unqualified
components of PRODUCT_BOOT_JARS get "platform:" prepended to them
after reading the product makefiles.

Test: aosp_walleye-userdebug boots
Bug: 138994281
Change-Id: I0f79c7d10477880ca65354251a5d1ca0b7ce79ab
diff --git a/core/soong_java_prebuilt.mk b/core/soong_java_prebuilt.mk
index 1496d56..509e3f6 100644
--- a/core/soong_java_prebuilt.mk
+++ b/core/soong_java_prebuilt.mk
@@ -91,7 +91,8 @@
 
 ifdef LOCAL_SOONG_DEX_JAR
   ifndef LOCAL_IS_HOST_MODULE
-    ifneq ($(filter $(LOCAL_MODULE),$(PRODUCT_BOOT_JARS)),)  # is_boot_jar
+    boot_jars := $(foreach pair,$(PRODUCT_BOOT_JARS), $(call word-colon,2,$(pair)))
+    ifneq ($(filter $(LOCAL_MODULE),$(boot_jars)),) # is_boot_jar
       ifeq (true,$(WITH_DEXPREOPT))
         # For libart, the boot jars' odex files are replaced by $(DEFAULT_DEX_PREOPT_INSTALLED_IMAGE).
         # We use this installed_odex trick to get boot.art installed.