Integrate dexpreopt into the build system.

Change-Id: Id67f85d0f5c8674f5bc22e431114ca73625811ef
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index c046c69..9681364 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -97,6 +97,7 @@
 LOCAL_EMMA_COVERAGE_FILTER:=
 LOCAL_MANIFEST_FILE:=
 LOCAL_BUILD_HOST_DEX:=
+LOCAL_DEX_PREOPT:=
 
 # Trim MAKEFILE_LIST so that $(call my-dir) doesn't need to
 # iterate over thousands of entries every time.