[Autofill] Pull ScopedAutofillManagersObservation out of content/

This CL moves ScopedAutofillManagersObservation from
//components/autofill/content/browser to
//components/autofill/core/browser/foundations and thus makes it
available on iOS.

It doesn't move the unit test because we don't have a
TestAutofillDriverFactory yet that'd suffice for the test.

Bypass-Check-License: Renamed files.
Bug: 445023368, 40178290
Change-Id: I0193c0a1cdf8335cce8a1e3420f2649ed5832ecd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6943178
Reviewed-by: Jan Keitel <jkeitel@google.com>
Commit-Queue: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/main@{#1515378}