[ios] Password KA: Throttle refresh of suggestions, first step

First step to do less useless suggestion refreshes in the KA for
form input suggestions.

Throttling rules introduced in this CL:
* Do not refresh if the app is in the background to not slowdown
backgrounding to avoid being terminated by the watchdog.
* Do not refresh when changing the suggestionEnabled bit unless
it was truly enabled (NO -> YES).

Change-Id: Ifc2d878e5fcd265c288e7f39d38befba6fb1f441
Bug: 365097975
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5919676
Commit-Queue: Vincent Boisselle <vincb@google.com>
Reviewed-by: Tommy Martino <tmartino@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1367041}