ash: Fix launcher apps page getting stuck in an invisible state

When triggering the show animation we need to abort any in-progress
hide animation. If the animations are triggered quickly then this
can happen:
* Hide animation starts
* Show animation starts, calls SetVisible(true)
* Hide animation finishes, calls SetVisible(false)
Then the page gets stuck in an invisible state.

Bug: 1295794
Test: added to ash_unittests
Change-Id: Iedab202372bbc4dc25ffa34201b49e9ed1a4771d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3451038
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/main@{#969490}