Add API to retrieve ElementIdentifier from app ID.

This API will be used to register AppListItemViews and
ShelfAppButtons for the Explore app and the Settings app with the
element tracker framework. This will enable them to serve as anchors
for help bubbles in the Welcome Tour.

Bug: b:275616974
Change-Id: I30dca4eb91bb226061a58a384b87e936e139ecfc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4563365


Reviewed-by: Andrew Xu <andrewxu@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/main@{#1149423}