ash: Remove StubDelegate from SessionManagerClient

Remove StubDelegate and SetStubDelegate from SessionManagerClient and
FakeSessionManagerClient.

In production, SessionManagerClient::RequestLockScreen forwards a lock
request to the session_manager daemon, which later calls back into
ScreenLockServiceProvider. For testing, ScreenLockObserver previously
used StubDelegate to intercept RequestLockScreen and directly show
the lock screen.

This change replaces StubDelegate with a repeating callback on
FakeSessionManagerClient (set_on_request_lock_screen_callback) and
introduces ScreenLockerTester::ScopedRequestLockScreenOverride to set
up this callback during tests. Callsites that previously depended on
StubDelegate or manual lock handling have been updated to use
ScreenLockerTester::Lock() or set the callback explicitly.

TAG=agy

Bug: b:539761804
Change-Id: I02e83282cac1f0717e1bad9612d91c2caba28b1d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8150800
Commit-Queue: Jun Ishiguro <junis@google.com>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1670986}