Restrict glic fre_status check to non-iOS builds

Make the glic fre_status check in
PersonalContextEligibilityServiceImpl::SatisfiesPrefsRequirements() and
its corresponding pref observer registration only available in non-iOS
builds using #if !BUILDFLAG(IS_IOS).

IOS uses a different pref to query the glic first run state.

Bug: 535537433
Change-Id: I80d3a05439940f99fa115f9d70082dc9c4e9b4a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8180386
Commit-Queue: Norge Vizcay <vizcay@google.com>
Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1671896}