[iOS][FRE] Activity loading UI when signing in

This patch adds API to be able to block the UI in the FRE sign-in
screen. This patch is required by crrev.com/c/3143970.

FRE disabled:
http://shortn/_1LqUNfVGD5

Bug: 1245577
Change-Id: I758f7b9e333bcdfe69c192e006aaeab10c080e63
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3141997


Auto-Submit: Vincent Boisselle <vincb@google.com>
Reviewed-by: Jérôme <jlebel@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Vincent Boisselle <vincb@google.com>
Cr-Commit-Position: refs/heads/main@{#918764}