ash: Cache the app list bubble widget for productivity launcher

After the first Show(), hide the widget instead of destroying it.
This is similar to how tablet mode launcher works. This eliminates
~30ms of latency for showing the launcher on eve.

Switch to using "target visibility" for IsShowing(). Make show and
hide reversible (rapidly clicking the button cancels the previous
animation and triggers the reverse one).

Bug: 1277676
Test: updated ash_unittests
Change-Id: Ibb8ebc7fef2d733681702b2ae0da11df4c135c23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3328736
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/main@{#951494}