[Autofill] Move FocusStateNotifier (back) to PasswordAutofillAgent

FocusStateNotifier was moved from PasswordAutofillAgent as a step
towards moving focus handling for the keyboard accessory to Autofill.
Since the browser-side code will remain in Password Manager for the
foreseeable future, this CL moves FocusStateNotifier back to
PasswordAutofillAgent

This way, we avoid that AutofillAgent fires
- AutofillDriver::FocusOn[Non]FormField() and
- PasswordManagerDriver::FocusedInputChanged()
which are fired in different situations:
- AutofillDriver::FocusOn[Non]FormField() is fired in a
  RenderFrameObserver::FocusedElementChanged() handler.
- PasswordAutofillDriver::FocusedInputChanged() is fired in
  WebAutofillClient::DidCompleteFocusChangeInFrame() and
  WebAutofillClient::TextFieldDidEndEditing() handlers.
  The former isn't fired when the focus moves out of the frame.
- The PWM event is treats contenteditables like non-fields.

A followup will try to migrate
PasswordAutofillDriver::FocusedInputChanged() to
RenderFrameObserver::FocusedElementChanged().

Bug: 40260756
Change-Id: I765b5733712eef2a6407a0dbdc9476c07abc44eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5893208
Reviewed-by: Jihad Hanna <jihadghanna@google.com>
Reviewed-by: Anna Tsvirchkova <atsvirchkova@google.com>
Commit-Queue: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/main@{#1361768}