Use prebuilt dx for PDK builds

The source for dx may not be available in PDK builds, use the
prebuilt one from prebuilts/build-tools instead.

Bug: 67663308
Test: m TARGET_BUILD_PDK=true
Change-Id: I9090b5190539f901fc05264a472133c12d4ea2a1

java/config/config.go

@@ -83,7 +83,7 @@ func init() {
 		if config.(android.Config).Getenv("USE_D8") == "true" {
 			dexer = "d8"
 		}
-		if config.(android.Config).UnbundledBuild() {
+		if config.(android.Config).UnbundledBuild() || config.(android.Config).IsPdkBuild() {
 			return "prebuilts/build-tools/common/bin/" + dexer, nil
 		} else {
 			path, err := pctx.HostBinToolPath(config, dexer)