[iOS] Insert item based on existing item instead of index

Replace the index by a grid item identifier to insert the new element

See video:
https://drive.google.com/file/d/1ZJf5KJRfZ8z6b967wzTz1MJg-qolqIvt/view?usp=drive_link

Bug: 1501837
Change-Id: Id35bcc45041264f1abf79a18ecd76f5a922dd012
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5383453


Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Aliona Dangla <alionadangla@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280195}