List variables needed to be percolated in the order of inherit() calls.
children.keys() was in that order, due to starlark dictionaries being
iterable in the order of insertion, but the previous cl broke that
behavior by sorting them. Instead, only sort the children for
single value variables.
Fixes: 226206409
Fixes: 228044099
Test: ./out/rbcrun ./build/make/tests/run.rbc and testing aosp_arm64
Change-Id: I5b91514e87b158b615e4d4ec7868fccb0248379b
Single-value product variables resolve to the first
instance of that variable in prefix order. What we
have right now is taking the value from the current
makefile if it's set, but if it's not, we take it
from the last child, not the first.
Fixes: 226206409
Test: Manually
Change-Id: Id23f7c269ff9a352bf0b67cb57156b72449f978e
