Simplify Workspace::constrainedStackingOrder()

Workspace::constrainedStackingOrder() is a constant source of issues.
This change replaces the current logic that builds the stacking order to
hopefully make constrainedStackingOrder() less crashy.

With the new version, constrainedStackingOrder() doesn't perform layer
caching. On the other hand, it's quite rare to see apps in the wild that
have more than just 1 or 2 members in the window group.

BUG: 436095