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: I035ad32233e49e2a30ce11f6c7c318a648511ef8
diff --git a/dexpreopt/testing.go b/dexpreopt/testing.go
index 147a562..b1fbef5 100644
--- a/dexpreopt/testing.go
+++ b/dexpreopt/testing.go
@@ -88,6 +88,15 @@
 	FixtureModifyGlobalConfig(func(android.PathContext, *GlobalConfig) {}),
 )
 
+var PrepareForTestWithDexpreoptConfig = android.GroupFixturePreparers(
+	android.PrepareForTestWithAndroidBuildComponents,
+	android.FixtureModifyContext(func(ctx *android.TestContext) {
+		ctx.RegisterParallelSingletonType("dexpreopt-soong-config", func() android.Singleton {
+			return &globalSoongConfigSingleton{}
+		})
+	}),
+)
+
 // FixtureModifyGlobalConfig enables dexpreopt (unless modified by the mutator) and modifies the
 // configuration.
 func FixtureModifyGlobalConfig(configModifier func(ctx android.PathContext, dexpreoptConfig *GlobalConfig)) android.FixturePreparer {
@@ -195,3 +204,10 @@
 		dexpreoptConfig.DisablePreopt = disable
 	})
 }
+
+// FixtureSetEnableUffdGc sets the EnableUffdGc property in the global config.
+func FixtureSetEnableUffdGc(value string) android.FixturePreparer {
+	return FixtureModifyGlobalConfig(func(_ android.PathContext, dexpreoptConfig *GlobalConfig) {
+		dexpreoptConfig.EnableUffdGc = value
+	})
+}