Enable ART source build.

This is the default when the make config step is run, but we need to do
it explicitly here since soong.variables is hardcoded.

Test: env OUT_DIR=out prebuilts/build-tools/build-prebuilts.sh
  on the aosp-build-tools branch
Bug: 188922057
Change-Id: I1f85c01b76a212b5cdd1332fb2445b184f4ad759
diff --git a/build-prebuilts.sh b/build-prebuilts.sh
index 00d0928..d80f37c 100755
--- a/build-prebuilts.sh
+++ b/build-prebuilts.sh
@@ -51,6 +51,9 @@
     "VendorVars": {
         "cpython3": {
             "force_build_host": "true"
+        },
+        "art_module": {
+            "source_build": "true"
         }
     }
 }
@@ -163,7 +166,12 @@
 {
     "Allow_missing_dependencies": true,
     "HostArch":"x86_64",
-    "SanitizeHost": ["address"]
+    "SanitizeHost": ["address"],
+    "VendorVars": {
+        "art_module": {
+            "source_build": "true"
+        }
+    }
 }
 EOF