Desk Templates: Allow updating and deleting grid items

There are currently too many calls to
DesksTemplatesPresenter::GetAllEntries, which essentially destroys and
recreates the entire grid whenever we have an update to the desk model.
This is unnecessarily expensive.

This CL fixes this issue by allowing us to add and delete individual
template item views without having to recreate the grid from scratch
every time. This is accomplished by finding the views by UUID and taking
an action accordingly.

This CL also fixes the issue of having unnecessary layouts when clicking
the desk templates button.

Bug: 1282035, 1266552
Test: manual, updated existing tests.
Change-Id: I8065630fdb4bb0783749cef7e0338ae4eaf14672
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3416651
Auto-Submit: Richard Chui <richui@chromium.org>
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Richard Chui <richui@chromium.org>
Cr-Commit-Position: refs/heads/main@{#965905}