[LR] Handle screenshot and download errors

Adds error handling for the cases where a screenshot cannot be taken on
the Lightweight Reactions dialog opening, or where the assets (GIFs
and thumbnails) fail to download to disk the first the feature is used.
In case of error, the dialog isn't launched, and a toast message is
showed instead.


The dialog view is now initialized and showed in
maybeFinishInitialization, to prevent showing the dialog and
immediately dismissing it in case of an asset fetch error. As a result,
the onViewCreated method has been removed in favor of a lambda.

Also adds a metric to record how long the asset fetch took before
failing.

While at it, a callback variable was renamed to remove some
confusion.

Bug: 1268473, 1268474
Change-Id: Idd3143fdd0abaefadbd7b368c03e7bdf0a2bb954
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3297934
Commit-Queue: Guillaume Jenkins <gujen@google.com>
Reviewed-by: sebsg <sebsg@chromium.org>
Reviewed-by: Sophey <sophey@chromium.org>
Cr-Commit-Position: refs/heads/main@{#945051}