Propagate the Soong config variable for enabling ART Module building.

It is set in art/build/build-art-module.sh.

Test: art/build/build-art-module.sh
Bug: 172480617
Change-Id: Idbb3478667a2265ba115922207cb6d9aa6c4c91d
diff --git a/scripts/build-aml-prebuilts.sh b/scripts/build-aml-prebuilts.sh
index 0c868ea..de22c45 100755
--- a/scripts/build-aml-prebuilts.sh
+++ b/scripts/build-aml-prebuilts.sh
@@ -94,7 +94,13 @@
 
     "Allow_missing_dependencies": ${SOONG_ALLOW_MISSING_DEPENDENCIES:-false},
     "Unbundled_build": ${TARGET_BUILD_UNBUNDLED:-false},
-    "UseGoma": ${USE_GOMA}
+    "UseGoma": ${USE_GOMA},
+
+    "VendorVars": {
+        "art_module": {
+            "source_build": "${ENABLE_ART_SOURCE_BUILD:-false}"
+        }
+    }
 }
 EOF