[User Education] Move IPH controller API down into UE service

This moves the API to set/get the controller (mostly for testing, as
almost all production calls should go through
BrowserUserEducationInterface) down into UserEducationService, which
actually owns the controller.

Bug: 369887569

Change-Id: I72e15efdbf48177726bc8843826c3fdc72f52485
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6777319
Auto-Submit: Dana Fried <dfried@chromium.org>
Reviewed-by: Christopher Grant <cjgrant@chromium.org>
Commit-Queue: Christopher Grant <cjgrant@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1493488}