Preserve relative order of transient siblings

If a constraint indicates that window A must be below window B but it's
not the case at the moment, the workspace will move window A right after
window B.

This can invert the relative order of transient siblings, for example
let's say that there are three constraints

- A <- B (window A must be below window B)
- A <- C
- A <- D

and the unconstrained stacking order looks as follows: [B, C, D, A]. The
final constrained stacking order is expected to look as [A, B, C, D],
but currently it's [A, D, C, B] instead:

- starting stacking order: [B, C, D, A]
- apply A <- B constraint: [C, D, A, B]
- apply A <- C constraint: [D, A, C, B]
- apply A <- D constraint: [A, D, C, B]

In order to fix this issue, this patch makes the workspace traverse the
constraint graph in the reverse order. In addition to that, it ensures
that the relative order of transient siblings in unconstrained stacking
order is preserved in the constrained one.

BUG: 477262


(cherry picked from commit 15b8fbe6)