Ash: Fix z-order with multi-display pinned windows

Fix an edge case where a pinned window in a multi-display setup will get
activated on desk switch, even if it is not the most recently used
window.

Also formatted desks_unittests.cc.

BUG=b:274110274
TEST=Updated existing tests to support multiple root windows

Change-Id: I998fee6d792533d7b2580a92efc9f81dcdd19445
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4546868


Quick-Run: Richard Chui <richui@chromium.org>
Reviewed-by: Daniel Andersson <dandersson@chromium.org>
Commit-Queue: Richard Chui <richui@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1147551}