Use correct observer in WebKioskAppUpdateObserverTest

Previously the tests all relied on a `AppRegistryCache::Observer` to
know when the app data is updated, but that is actually incorrect
since that only guarantees the WebKioskAppUpdateObserver received a
*request* to update the app data, it doesn't guarantee that it actually
processed and finished this request.

So now the tests rely on the KioskAppManagerObserver, which is invoked
only after the update request is processed and finished

Bug: N/A
Test: unittests
Change-Id: I0daa29693278e6471b608f6783bb9b6f7598b04e
Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4161897
Commit-Queue: Ben Franz <bfranz@chromium.org>
Auto-Submit: Jeroen Dhollander <jeroendh@google.com>
Commit-Queue: Jeroen Dhollander <jeroendh@google.com>
Reviewed-by: Ben Franz <bfranz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1093030}