Add support for single-value product variables

Prior to this change, all PRODUCT_* variables are inherited with
list semantics, meaning the variable value in the current makefile
as well as all the makefiles it inherits are concatenated together.

Many of these variables are not lists, so this change adds the
ability to classify a variable as a single-value variable, which
will inherit at most one value (the first one), unless the variable
is already defined in the current file.

This change also marks all current variables as list variables.
A future change will re-classify the single-value variables as such.

Bug: 116769560
Test: presubmit
Change-Id: I57719a5eefd0749e92c08b1773b6f929629267a3