Clean up AttributionCookieChecker usage

- Introduce callback alias
- Remove deferred callback from queue before invoking in
  MockCookieChecker in order to avoid a situation in which synchronous
  recursion could end up invoking the same callback twice
- Simplify callback usage in AttributionCookieCheckerImpl using Then

A follow-up change will simplify MockCookieChecker because it is
currently unrealistic: Based on how AttributionManagerImpl works, there
can never be multiple deferred callbacks, but MockCookieChecker (and
tests using it) operate under the assumption that this is possible.

Bug: 330745162
Change-Id: I44c88d36f15b29417c9a14c505008337b846b057
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5405462
Reviewed-by: Nan Lin <linnan@chromium.org>
Commit-Queue: Andrew Paseltiner <apaseltiner@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280157}