[SxS] Hide drop targets immediately when drag is out of drop zone

This addresses a potential race condition, where moving a drag out of
the drop zone, while the drag target is animating in, but before the
mouse is over it, will keep the drop target view visible.

The reason is that the drop controller expected the view to handle the
drag (and hide itself) once it is animating in.

Change-Id: I44c0be93d47fca8de1a36feb77b372879ce7feac
Bug: 433720670
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6792229
Reviewed-by: Caroline Rising <corising@chromium.org>
Commit-Queue: Kaan Alsan <alsan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1493628}