Make the PayDM an identity observer

The PDM's identity observer is only used for payments-specifics logic.
This CL removes the identity observer from the PDM and observes the
identity manager from the PayDM instead.

In the PayDM, a ScopedObservation is used, which avoid the
RemoveObserver() calls scattered around the PDM.

The implementation of PDM::OnAccountsCookieDeletedByUserAction() and the
corresponding test are moved without changes.

Bug: b/322170538
Change-Id: I0ac96ec750c35350a9661a1c151952fd14f1383c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5439379
Commit-Queue: Florian Leimgruber <fleimgruber@google.com>
Reviewed-by: Jihad Hanna <jihadghanna@google.com>
Cr-Commit-Position: refs/heads/main@{#1285867}