FLEDGE: Move interest group management from AdAuction to AuctionRunner.

We're working on merging these two classes, now that they're both in
the browser process, and there's not much to gain from separating them.

This also makes the logic testable outside of browser tests, since
AuctionRunner has a unit test fixture, while AdAuction does not.

Bug: 1205517
Change-Id: I51aee59932d710e35d0332f6a122987e6f0ad05d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2905192
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Maksim Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#884725}