[ios] Introduce some histograms for background customization gallery

Adds the following UMA metrics to monitor some iOS NTP background
customization gallery:
- IOS.HomeCustomization.Background.Gallery.CollectionImageFetchError:
Logs the type of error when fetching images urls within a collection.
- IOS.HomeCustomization.Background.Gallery.CollectionInfoFetchError:
Logs the type of error when fetching the list of background collections.
- IOS.HomeCustomization.Background.Gallery.ImageDownloadSuccessful:
Records a boolean indicating whether an individual image was successfully downloaded.

Bug: 445134466
Change-Id: I81d4b815cc00aab81be00ebea9fc9faf824d634e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6954789
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Commit-Queue: Guillaume Jenkins <gujen@google.com>
Reviewed-by: Guillaume Jenkins <gujen@google.com>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1518691}