Clean the build.prop so that default notification sound is set properly.

Bug: 3349429
Change-Id: I44450a4cc9acdeffa42340c61088a5964fa6ee0d
diff --git a/CleanSpec.mk b/CleanSpec.mk
index 0e89c6a..6d6dd5b 100644
--- a/CleanSpec.mk
+++ b/CleanSpec.mk
@@ -64,6 +64,7 @@
 $(call add-clean-step, rm -rf $(PRODUCT_OUT)/obj/APPS/*)
 $(call add-clean-step, rm -rf $(PRODUCT_OUT)/system/build.prop)
 $(call add-clean-step, rm -rf $(PRODUCT_OUT)/obj/SHARED_LIBRARIES/*/LINKED)
+$(call add-clean-step, rm -rf $(PRODUCT_OUT)/system/build.prop)
 
 # ************************************************
 # NEWER CLEAN STEPS MUST BE AT THE END OF THE LIST