Add DEXPREOPT support for ART

Change-Id: I24d0d7b2a23a769f5d69bd4dc14be22e1475b759
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 0a3517b..0648b35 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -119,6 +119,7 @@
 LOCAL_RENDERSCRIPT_TARGET_API:=
 LOCAL_BUILD_HOST_DEX:=
 LOCAL_DEX_PREOPT:= # '',true,false,nostripping
+LOCAL_DEX_PREOPT_IMAGE:=
 LOCAL_PROTOC_OPTIMIZE_TYPE:= # lite(default),micro,nano,full
 LOCAL_PROTOC_FLAGS:=
 LOCAL_PROTO_JAVA_OUTPUT_PARAMS:=