[iOS]Restart signin coordinator only if needed

Due to a bug in UIKit up to iOS18, the Add Account view may disappear
without calling its callback. Furthermore, many coordinators can
directly open the add account (or reauth) view on top of
self.baseViewController. This mean that, when we end-up with a request
to start a sign-in coordinator (directly or indirectly) and a sign-in
coordinator is already started, it’s impossible to know whether this is
a double tap and the request should be ignored. Or whether the view
disappeared silently, and so the previous coordinator should be stopped
and a new one started.

Stopping and restarting is generally speaking a bad idea, as it may mean
interrupting the user during an authentication in progress, or even
having UIKit warns that we should not have done UI change before the
previous UI was change was done. So this CL ensure that the already
started coordinator is stopped in case of doubt. And if the program can
be certain that the view is still displayed, the request for a new
coordinator is simply ignored.

This CL introduce the method `ShouldStartANewCoordinator` that informs,
depending on the value assigned currently to the coordinator variable
whether a new coordinator should be started. This allows to simplify
both the content of the `if` and the place where the logic for the early
return is explained.

In order to do so, it also introduces a protocol that informs whether
the coordinator can have this bug. This protocol will become even more
useful when more coordinators stops inheriting from SigninCoordinator.

Bug: 395959814
Change-Id: I6b89c836142ab905c4d76fd6a6cfdae02426fd07
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6818278
Commit-Queue: Mark Cogan <marq@chromium.org>
Auto-Submit: Arthur Milchior <arthurmilchior@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1503385}