Enforce that PRODUCT_ variables aren't modified by module makefiles

This eliminates one of the way a module could insert itself into
a build without going through a product definition file.

Change-Id: I1d537764d73c52ceaec96b64b6b6925e2f6a8d8e
diff --git a/core/main.mk b/core/main.mk
index 8172b22..9efdc5d 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -474,7 +474,7 @@
 # Before we go and include all of the module makefiles, stash away
 # the PRODUCT_* values so you can't get to them.
 ifeq ($(stash_product_vars),true)
   $(call stash-product-vars, __STASHED, DO_NOT_USE_IN_ANDROID_MK_)