desks: Do not recreate layers for desk slide out animation

The large amounts of layers in the desks bar makes the recursive
`wm::RecreateLayers` function take too much time. So we cannot afford
to do it at this point.

This means we need to keep `DeskBarViewBase` alive until the animation
is finished. With the new raw_ptr, this causes `root` to be dangling if
a display is removed while the animation is running. We pass
ownership to `CleanupAnimationObserver`, which is owned by
`OverviewController`, and clean up the object if the root is removed
during its lifetime.

Test: existing tests
Test: manual
Change-Id: Ia34c12591dd7f7465a0d22258bf82d009d5a22b1
Fixed: b/294448300
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4753809


Reviewed-by: Yongshun Liu <yongshun@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1180439}