chromeos: Remove DebugDaemonClient from DBusThreadManager, part 2

This simplifies the DBusThreadManager and DBusClientsBrowser classes.
This is a follow-up to a migration from the mustash era (circa 2019)
where we're moving D-Bus clients to each have their own initialization
and shutdown. See bug.

This CL removes DBusThreadManager::GetDebugDaemonClient() and
migrates callers to DebugDaemonClient::Get().

Bug: 647367
Test: existing ash_components_unittests, unit_tests and browser_tests
Change-Id: Iad04d257db5d00c79f7ca7c4d9c7ca2a3f411ce9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3771020
Reviewed-by: Roger Tawa <rogerta@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1026806}