[iOS] Fixing AutoDeletionIPH Dismiss Button for iOS26

The AutoDeletionIPHViewController's dismiss button was misconfigured for
iOS26' Liquid Glass aesthetic. To address this, this CL replaces the
VC's custom dismiss button with the UIBarButtonSystemItemClose system
button.

After the misconfigured dismiss button was fixed, a new issue was
observed. The dismiss button no longer had the proper spacing between
the top of the half-sheet UI and the button. The CL fixes this by
decoupling the AutoDeletionIPHVC from its ConfirmationAlertController
and adding padding between the two views.

Fixed: 433720924
Change-Id: I24f05096a94f52c693923036502964c90cb7c74c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6977024
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Daniel White <danieltwhite@google.com>
Auto-Submit: Daniel White <danieltwhite@google.com>
Cr-Commit-Position: refs/heads/main@{#1520606}