Conform to convention for build env var for small art

Change-Id: Ic1f8c490a9341f2526713aa9b9b6c7ff9d4a8c70
diff --git a/build/Android.common.mk b/build/Android.common.mk
index e06c8cd..43f7ff5 100644
--- a/build/Android.common.mk
+++ b/build/Android.common.mk
@@ -19,6 +19,9 @@
 $(info Enabling ART_SMALL_MODE because of existence of art/SMALL_ART)
 ART_SMALL_MODE := true
 endif
+ifeq ($(WITH_ART_SMALL_MODE), true)
+ART_SMALL_MODE := true
+endif
 
 ART_USE_PORTABLE_COMPILER := false
 ifneq ($(wildcard art/USE_PORTABLE_COMPILER),)