[iOS] Introduce feature UseAccountListFromIdentityManager

We want to migrate from a bunch of (iOS-specific)
ChromeAccountManagerService APIs to corresponding (cross-platform)
IdentityManager APIs, see linked bug. The changes to do this were
already implemented, behind the feature flag
SeparateProfilesForManagedAccounts which also does a bunch of other
things.

For an easier and cleaner rollout, this CL introduces a new feature
specifically for the API change. It also converts one usage over to the
new feature, as an example.

Bug: 377467350
Change-Id: I933418ce54926c6f5c789e67c6b511af3a3865be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6190593
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1411655}