[iOS] Introduce IsIdentityDiscAccountMenuEnabled() helper

This should be used instead of querying the state of
kIdentityDiscAccountMenu directly.

The helper returns true if that feature is enabled, but also if
`AreSeparateProfilesForManagedAccountsEnabled()` is true, since that
feature depends on the account menu.

Bug: 383977223
Change-Id: I364948cac5ebe512950418172c206b61f9728475
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6382159
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1439358}