Change RRO default location to /product

This changes the default location of RRO packages that don't
specify where they should be installed to /product instead of
/vendor.

Also change the auto-generated "enforced RRO" packages to install
on /product, and be explicit about that in the mk to make it
independent of the RRO default.

The reasoning for this change is that most overlays tend to be
for product level customizations, like themes and so on. RROs
for hardware related things can still be explicitly installed
on /vendor.

Bug: 123566903
Test: preubmit
Test: various internal tests
Change-Id: Ie7d5f19dc7ab421a7bcf584153f991e18e961cda