Automate the strip and assignment of PRODUCT_ vars

With this change, all PRODUCT_ variables are treated the same
when it comes to stripping and assigning them to their final
variable name. In the past, all the PRODUCT variables needed
to be listed in two places to achieve this.

The documentation previously attached to the strip/assignment
is moved to the PRODUCT_ variable list in product.mk.

Also refactor some of the default value logic to cope with
the new automation.

Many places in the build system that currently refer to
$(PRODUCTS.$(INTERNAL_PRODUCT).X) can now be modified to
use $(X) directly.

Bug: 116769560
Test: verified noop on PRODUCT_ variables on all products in the tree
Change-Id: I5677c355e81359b1d3c0db2a2232941097a05047