Explicitly add ART boot images to build-art-target-golem.

Golem builds the ART module with `TARGET_BUILD_UNBUNDLED=true`. When we
move the boot image out of the ART APEX, it will no longer be built.
This change solves the problem by explicitly adding ART boot images to
build-art-target-golem.

Bug: 211973309
Test: Will see the result on Golem.
Change-Id: I86f2f0e4223192cb316f446a151dabf7790774a5
diff --git a/Android.mk b/Android.mk
index aa33c08..47d0a2b 100644
--- a/Android.mk
+++ b/Android.mk
@@ -711,6 +711,7 @@
                         $(TARGET_OUT_EXECUTABLES)/dex2oat_wrapper \
                         $(ART_TARGET_PLATFORM_DEPENDENCIES) \
                         $(ART_TARGET_SHARED_LIBRARY_BENCHMARK) \
+                        $(PRODUCT_OUT)/apex/art_boot_images/javalib/$(TARGET_ARCH)/boot.art \
                         standalone-apex-files
 	# remove debug libraries from public.libraries.txt because golem builds
 	# won't have it.