Break constant information out of product vars

We don't want rules to load the product variables directly, because
then they won't be able to transition on them.

Break constant information that is safe to load outside of the
product vars file, so that we can make the product vars file have
more restricted visibility later.

Bug: 269577299
Test: m nothing
Change-Id: I848bff33e4f5798f51296ea3a2600615cab36985