Fix single value variable inheritance order

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