desk_templates: Reduce amount of calls to the models.

- Add a check to make sure we only populate the grid once.
- Removes extra async call when adding or updating an element.
- Removes cached uuid for nudge purposes in favor of passing it
  through callbacks.

Test: existing tests
Change-Id: Ic83926fa602f7cb0634a26b401d5dc95d3bd4fd9
Fixed: 1299928
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3485438


Reviewed-by: Richard Chui <richui@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/main@{#975191}