[iOS] Keep SeparateProfiles enabled if there are multiple profiles

The feature kSeparateProfilesForManagedAccounts adds the ability to
create separate profiles.
If the feature got disabled after some profiles have been created, those
profiles would become inaccessible.

So this CL updates the logic in
AreSeparateProfilesForManagedAccountsEnabled() to keep returning true if
there are already multiple profiles, but the feature flag is off. It
also adds a separate kill switch feature flag, to turn it off
unconditionally.

Bug: 383977223
Change-Id: Icaea654274007985aff8d1056c90e47b9531099d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6408472
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1442258}