[iOS]Fix AddressBarPreferenceMediator PrefBackedBoolean not stopped before dealloc crash

This CL add the stop function call when the view controller was removed
from its parent so that the PrefBackedBoolean can be stopped before
AddressBarPreferenceMediator dealloc

Bug: 420310121
Change-Id: I7b2d8b2297f3788d642d47e75c68ffcbe0d3a04d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6587326
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: Scott Yoder <scottyoder@google.com>
Auto-Submit: Tian Ye <tianye@microsoft.com>
Commit-Queue: Scott Yoder <scottyoder@google.com>
Cr-Commit-Position: refs/heads/main@{#1465943}