Always use installed version of d8-compat-dx

Also adapt to the renaming of d8 to d8-compat-dx.

Test: USE_D8=true m -j16 PRODUCT-aosp_arm64-eng showcommands core-oj
Change-Id: Ic7aaa6a5da34a6a5706c20f53922a83cb1de1100

java/config/config.go

@@ -80,14 +80,18 @@ func init() {
 	pctx.HostBinToolVariable("SoongZipCmd", "soong_zip")
 	pctx.HostBinToolVariable("MergeZipsCmd", "merge_zips")
 	pctx.VariableFunc("DxCmd", func(config interface{}) (string, error) {
-		dexer := "d8"
 		if config.(android.Config).IsEnvFalse("USE_D8") {
-			dexer = "dx"
-		}
-		if config.(android.Config).UnbundledBuild() || config.(android.Config).IsPdkBuild() {
-			return "prebuilts/build-tools/common/bin/" + dexer, nil
+			if config.(android.Config).UnbundledBuild() || config.(android.Config).IsPdkBuild() {
+				return "prebuilts/build-tools/common/bin/dx", nil
+			} else {
+				path, err := pctx.HostBinToolPath(config, "dx")
+				if err != nil {
+					return "", err
+				}
+				return path.String(), nil
+			}
 		} else {
-			path, err := pctx.HostBinToolPath(config, dexer)
+			path, err := pctx.HostBinToolPath(config, "d8-compat-dx")
 			if err != nil {
 				return "", err
 			}

root.bp

@@ -29,6 +29,7 @@ optional_subdirs = [
     "packages/apps/*",
     "prebuilts/clang/host/linux-x86",
     "prebuilts/ndk",
+    "prebuilts/r8",
     "prebuilts/sdk",
     "prebuilts/misc",
     "system/*",