Verify PRODUCT_* are not modified by Android.mks, but allow them to be read only.

Bug: 3076955
Change-Id: I7ddcc5b9d6325d3a3c63f952f97034ddf223630a
diff --git a/core/main.mk b/core/main.mk
index 8172b22..1c7706a 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -473,10 +473,10 @@
 endif	# !SDK_ONLY
 
 # Before we go and include all of the module makefiles, stash away
-# the PRODUCT_* values so you can't get to them.
-stash_product_vars:=#true
+# the PRODUCT_* values so that later we can verify they are not modified.
+stash_product_vars:=true
 ifeq ($(stash_product_vars),true)
-  $(call stash-product-vars, __STASHED, DO_NOT_USE_IN_ANDROID_MK_)
+  $(call stash-product-vars, __STASHED)
 endif
 
 ifneq ($(ONE_SHOT_MAKEFILE),)
@@ -510,8 +510,7 @@
 endif # ONE_SHOT_MAKEFILE
 
 ifeq ($(stash_product_vars),true)
-  $(call assert-product-vars, __STASHED, DO_NOT_USE_IN_ANDROID_MK_)
-  $(call restore-product-vars, __STASHED)
+  $(call assert-product-vars, __STASHED)
 endif
 
 include $(BUILD_SYSTEM)/legacy_prebuilts.mk