Always assign layer when restacking special windows.

In assignAndIncreaseLayerIfNeeded we need to always
restack the special windows, even if we are stacking them
downwards, otherwise they could be left at too high of
a value from a previous pass. This check was added by
me originally, as a now revealed poorly thought out attempt
to avoid changing existing behavior too much.

Bug: 28139028
Change-Id: I305499e194f2c6bcf7a38c80af1e64bd1fc20943