[ash] Move CheckGenAIAgeAvailability into //ash

CoralDelegate::CheckGenAIAgeAvailability existed only so //ash could
reach a user's IdentityManager and account capabilities from //chrome.
Now that ash::IdentityManagerProvider provides the IdentityManager, the
whole GenAI age check can live in //ash.

Move the logic (including the refresh-token-load observation and its
timeout) into BirchCoralProvider, which becomes a
signin::IdentityManager::Observer and resolves the IdentityManager via
ash::IdentityManagerProvider::Get().Find(). For now the active user's
AccountId comes from session_manager::SessionManager's active session,
with a TODO to take it from the caller. Delete the CoralDelegate
interface method, its //chrome implementation, and the test-delegate
override.

BirchCoralProvider is the first //ash caller of IdentityManagerProvider.
Its implementation lives in //chrome, a different shared library, so in
component builds the provider's process-wide instance was duplicated per
shared library and Get() saw a null instance. Make the provider a
component() so there is a single instance across libraries.

For unit tests, add a FakeIdentityManagerProvider (keyed by account id,
so Find() returns a manager only for accounts a test registers) and a
ScopedCoralGenAIAvailability helper that installs it backed by an
IdentityTestEnvironment whose account reports the GenAI capability, so
coral stays available in the birch and coral tests that enter overview.

Bug: 542786399
Test: ash_unittests

Change-Id: I60a4f719c889f5f1384b4aa6f01040c33be893bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8252954
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Tiago Vignatti <tvignatti@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1693430}