Fix proguard flag file dependency.

Change-Id: I167ded6f8ce3084960f507a99587b53966d607fc
diff --git a/Android.mk b/Android.mk
index a906cda..9d1aeae 100644
--- a/Android.mk
+++ b/Android.mk
@@ -34,7 +34,7 @@
 
 LOCAL_RESOURCE_DIR := $(LOCAL_PATH)/res
 
-LOCAL_PROGUARD_FLAGS := -include $(LOCAL_PATH)/proguard.flags
+LOCAL_PROGUARD_FLAG_FILES := proguard.flags
 
 include $(BUILD_PACKAGE)