Remove PDMObserver::OnPersonalDataSyncStateChanged()

Only the AutofillPrivateEventRouter is listening for this event. In an
effort to simplify the PDMObserver, as part of go/split-pdm, this CL
removes the event from PDMObserver and instead observes the sync service
from AutofillPrivateEventRouter directly.

As a drive-by clean-up, the existing PDMObserver in
AutofillPrivateEventRouter is refactored to a base::ScopedObservation<>.

Bug: b/322170538
Change-Id: I0ebd362fa78ee227a517ff8b6300e6d29ba66da0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5234272
Reviewed-by: Dmitry Vykochko <vykochko@google.com>
Commit-Queue: Florian Leimgruber <fleimgruber@google.com>
Cr-Commit-Position: refs/heads/main@{#1261590}