Remove usages of long-form variables

The long-form variables (PRODUCTS.<makefile>.<variable>)
are used to get information about multiple products.
However, they've never really worked correctly, and so
importing multiple products is deprecated behavior.

Remove as many usages of the long-form variables and
multi-product imports as possible.

Bug: 228518445
Test: Manually
Change-Id: I0b67f16360ff8bdcdb39638de739440472bccf76