Determine GC type based on BUILT_KERNEL_VERSION_FILE.

How it works:
1. build/make/core/Makefile generates a txt file with the kernel
   version, which is taken from an explicit BOARD_KERNEL_VERSION value,
   or extracted from the kernel image on the source tree, or extracted
   from the kernel image extracted from the prebuilt boot.img.
   The file is saved at
   $ANDROID_PRODUCT_OUT/obj/PACKAGING/check_vintf_all_intermediates/kernel_version.txt.
2. If PRODUCT_ENABLE_UFFD_GC is "default", meaning the GC type needs to
   be determined by the kernel version, build/make/core/Makefile copies
   kernel_version.txt to
   out/soong/dexpreopt/kernel_version_for_uffd_gc.txt.
3. build/soong/dexpreopt/config.go writes the the UFFD GC flag to
   out/soong/dexpreopt/uffd_gc_flag.txt. The flag is determined by an
   explicit PRODUCT_ENABLE_UFFD_GC value or by contruct_uffd_gc_flag.py,
   which reads kernel_version_for_uffd_gc.txt and determines the flag
   accordingly.
4. dex2oat takes the UFFD GC flag from uffd_gc_flag.txt.
5. post_process_props.py mangles ro.dalvik.vm.enable_uffd_gc based on
   the same logic.

Bug: 321751629
Bug: 319554951
Bug: 318763448
Bug: 319648491
Test: m --no-skip-soong-tests nothing
Test: atest uffd_gc_utils_test
Test: Build with `OVERRIDE_ENABLE_UFFD_GC=default m` for device with no
  UFFD support -
  1. Check the existence of `-Xgc:CMC` in
     out/soong/dexpreopt_arm64/dex_bootjars/android/system/framework/arm64/boot.invocation
     (dex2oat invocation for a boot image)
  2. Check the existence of `-Xgc:CMC` in
     out/soong/.intermediates/packages/apps/Settings/Settings/android_common/dexpreopt/Settings/oat/arm64/package.invocation
     (dex2oat invocation for an app defined in .bp)
  3. Check the existence of `-Xgc:CMC` in
     $ANDROID_PRODUCT_OUT/obj/APPS/Dialer_intermediates/oat/arm64/package.invocation
     (dex2oat invocation for an app defined in .mk)
  4. Check the value of ro.dalvik.vm.enable_uffd_gc in
     $ANDROID_PRODUCT_OUT/product/etc/build.prop
Test: Build with `OVERRIDE_ENABLE_UFFD_GC=default m` for device with
  UFFD support, and do the steps above.
Test: Build with `OVERRIDE_ENABLE_UFFD_GC=true m`, and do the steps
  above.
Test: Build with `OVERRIDE_ENABLE_UFFD_GC=false m`, and do the steps
  above.

Change-Id: I8df6e5be1644da05d2d5c57b3520f29601dfd7a4
diff --git a/core/dex_preopt.mk b/core/dex_preopt.mk
index 37a389f..08311ca 100644
--- a/core/dex_preopt.mk
+++ b/core/dex_preopt.mk
@@ -57,6 +57,7 @@
 # Build the boot.zip which contains the boot jars and their compilation output
 # We can do this only if preopt is enabled and if the product uses libart config (which sets the
 # default properties for preopting).
+# At the time of writing, this is only for ART Cloud.
 ifeq ($(WITH_DEXPREOPT), true)
 ifneq ($(WITH_DEXPREOPT_ART_BOOT_IMG_ONLY), true)
 ifeq ($(PRODUCT_USES_DEFAULT_ART_CONFIG), true)
@@ -95,15 +96,16 @@
 bootclasspath_locations_arg := $(subst $(space),:,$(DEXPREOPT_BOOTCLASSPATH_DEX_LOCATIONS))
 boot_images := $(subst :,$(space),$(DEXPREOPT_IMAGE_LOCATIONS_ON_DEVICE$(DEXPREOPT_INFIX)))
 boot_image_arg := $(subst $(space),:,$(patsubst /%,%,$(boot_images)))
-dex2oat_extra_args := $(if $(filter true,$(ENABLE_UFFD_GC)),--runtime-arg -Xgc:CMC)
+uffd_gc_flag_txt := $(OUT_DIR)/soong/dexpreopt/uffd_gc_flag.txt
 
 boot_zip_metadata_txt := $(dir $(boot_zip))boot_zip/METADATA.txt
+$(boot_zip_metadata_txt): $(uffd_gc_flag_txt)
 $(boot_zip_metadata_txt):
 	rm -f $@
 	echo "bootclasspath = $(bootclasspath_arg)" >> $@
 	echo "bootclasspath-locations = $(bootclasspath_locations_arg)" >> $@
 	echo "boot-image = $(boot_image_arg)" >> $@
-	echo "extra-args = $(dex2oat_extra_args)" >> $@
+	echo "extra-args = `cat $(uffd_gc_flag_txt)`" >> $@
 
 $(call dist-for-goals, droidcore, $(boot_zip_metadata_txt))