Add USE_D8_DESUGAR option in build/soong

Add an option to use D8's desugar instead of the standalone version.

USE_D8_DESUGAR=true m would trigger DCHECK in dex2oat unless
https://android-review.googlesource.com/c/platform/art/+/562595 is
patched in as well.

Bug: 69329508
Test: m && USE_D8_DESUGAR=false m

Change-Id: I864d88e257a2ba0b7f19aa5cced537301950e963

java/config/config.go

@@ -106,6 +106,13 @@ func init() {
 			return path.String(), nil
 		}
 	})
+	pctx.VariableFunc("D8Cmd", func(config android.Config) (string, error) {
+		path, err := pctx.HostBinToolPath(config, "d8")
+		if err != nil {
+			return "", err
+		}
+		return path.String(), nil
+	})
 	pctx.VariableFunc("TurbineJar", func(config android.Config) (string, error) {
 		turbine := "turbine.jar"
 		if config.UnbundledBuild() {