Ensure current product use PRODUCT_* vars directly

Disallow use of the long-form PRODUCTS.$(INTERNAL_PRODUCT).PRODUCT_*
after the short-form has been assigned, to ensure modifications are
made to the right variable.

Macros that need to work with multiple products get a convenience
macro that is redefined at the right moment.

Test: compare_target_files
Test: build_test
Change-Id: Ib0e57b1bc51b1f308296a150b9b7590a0bb5c313