arc: Convert Observers in ash/components/arc/session to CheckedObservers

I have changed following Observers to inherit base::CheckedObserver.
- ArcClientAdapter::Observer
- ArcSession::Observer
- ArcSessionRunner::Observer

Note that the ::UncheckedAndDanglingUntriaged tag has not been touched.

Bug: 40576343
Change-Id: Ied35774510af554913d3ea035ff9af4c477e74fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5753607
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Eriko Kurimoto <elkurin@chromium.org>
Commit-Queue: Soshun Naito <soshun@google.com>
Cr-Commit-Position: refs/heads/main@{#1336469}