ash: Refactor `DeskBarBoundsAnimation`

This CL removes the self-deleting class `DeskBarBoundsAnimation` and
replaces it with the function `AnimateDeskBarBounds`. The CL does
not change the functionality of the code.

This is [2/X] CL, and the goal is to rewrite all desk bar related
animations to use animation builder with abort handle.

It also adds an once closure to the desk bar view, so that in test
suites, it knows when desk bar UI updates are done.

Bug: N/A
Test: ash_unittests
Change-Id: I2da92d52a483611dd82a6470e88482ca80695c5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4701697
Reviewed-by: Daniel Andersson <dandersson@chromium.org>
Commit-Queue: Yongshun Liu <yongshun@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1173597}