Set the Soong config variable to keep building ART from source.

Without this Soong may run into inconsistencies due to stale prebuilts.
This amends https://r.android.com/1529658.

Test: TH build of NDK
Bug: 175918603
Bug: 172480615
Change-Id: I14680e4547d7d5f560d31ef61b71af8d159c4c53

scripts/build-ndk-prebuilts.sh

@@ -56,6 +56,12 @@ cat > ${SOONG_OUT}/soong.variables << EOF
     "Ndk_abis": true,
     "Exclude_draft_ndk_apis": true,
 
+    "VendorVars": {
+        "art_module": {
+            "source_build": "true"
+        }
+    },
+
     "MissingUsesLibraries": ${MISSING_USES_LIBRARIES}
 }
 EOF