Handle soong config vars for string attrs

Previously if there was a value set outside of product variables and one
inside a conditions default soong config var, we would ignore the
conditions default. Now, we maintain the conditions default and use
the root value in cases where there is not a value for a product
variable.

Test: go test soong tests
Change-Id: Ic991e3aebe5bb6039353f4e3d25625e7c5190f96