Add art

Re-land I557c86d2282178d215ea17abb71e556bdc4ba722

Change-Id: Ic26e1a6b17569acbfe68b4a219982ce45cecefbd
diff --git a/root.bp b/root.bp
index 4b03cf1..c4f2d57 100644
--- a/root.bp
+++ b/root.bp
@@ -6,6 +6,7 @@
 ]
 
 optional_subdirs = [
+    "art",
     "bionic",
     "build/kati",
     "build/tools/*",