Clean up /system/app -> /system/priv-app in incremental builds

Change-Id: Ib5596532e042e11cb1227d8a33367d24a3163431
diff --git a/CleanSpec.mk b/CleanSpec.mk
index 6d43bc0..461aff4 100644
--- a/CleanSpec.mk
+++ b/CleanSpec.mk
@@ -178,6 +178,9 @@
 $(call add-clean-step, rm -rf $(PRODUCT_OUT)/system/app/*)
 $(call add-clean-step, rm -rf $(PRODUCT_OUT)/obj/APPS/*)
 
+# Clean up around the /system/app -> /system/priv-app migration
+$(call add-clean-step, rm -rf $(PRODUCT_OUT)/system/app/*)
+
 # ************************************************
 # NEWER CLEAN STEPS MUST BE AT THE END OF THE LIST
 # ************************************************