Enable art for user builds

Bug: 10069754
Change-Id: Iaaff33a97bd7b68b2e8d84cd0034b15dde8a4316
diff --git a/core/java_library.mk b/core/java_library.mk
index 9dbd79b..4c6b652 100644
--- a/core/java_library.mk
+++ b/core/java_library.mk
@@ -40,7 +40,7 @@
 ifeq (,$(TARGET_BUILD_APPS))
 ifeq (,$(LOCAL_APK_LIBRARIES))
 ifndef LOCAL_DEX_PREOPT
-LOCAL_DEX_PREOPT := true
+LOCAL_DEX_PREOPT := $(DEX_PREOPT_DEFAULT)
 endif
 endif
 endif