[iOS] Fix new folder in account storage

This patch updates the code to create new folder in
BookmarksHomeViewController. The new folder is now created using
the storage of the displayed node.

This patch also creates the following property:
-BookmarksHomeMediator.displayedBookmarkModel
and the following method:
-[BookmarksHomeMediator shouldDisplayCloudSlashIconWithBookmarkModel:]

Fixed: 1434588, 1432759
Bug: 1428044
Change-Id: I93780af1a6d54b1e3367f212c989fac9be3b7fb1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4465950
Reviewed-by: Arthur Milchior <arthurmilchior@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Auto-Submit: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1138863}