[iOS] Fix overly removal when InstantSigninCoordinator is interrupted

If the instant sign-in is interrupted while the sign-in is in
progress, -[InstantSigninCoordinator stopAnimated:] is going to remove
the activity overlay, and the mediator will try to remove the activity
overlay too while being disconnected.
This is an issue since it was already removed.

The flow while the coordinator is interrupted is more complicated.
We can have reentrance:
 1. coordinator stop method calls mediator disconnect
 2. mediator disconnect calls coordinator didSigninWithResult method
 3. coordinator didSigninWithResult method calls signinCompletion
 4. signinCompletion calls the coordinator owner
 5, the coordinator owner calls the coordinator stop

Then, the coordinator owner dealloc the instant sign-in coordinator
while the coordinator stop is still in progress in stop 1.

The mediator's delegate can be remove before calling `[_mediator
disconnect]`. This will prevent 2. to happen.
Since the stop method everyone is aware that the sign-in is stopped.

Fixed: 421243213
Change-Id: I03b250cb2e4c7e2c1688ced7974c597f8afcde1f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6632577
Reviewed-by: Arthur Milchior <arthurmilchior@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Auto-Submit: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Arthur Milchior <arthurmilchior@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1473194}