Restructure randomized-response logic

- Return both rate and response from one method on delegate: This
  reduces 3 GetNumberOfStarsAndBarsSequences calls per source to 1.
- Perform channel-capacity check in delegate instead of storage: This
  simplifies the storage code.
- Move pure-math code from delegate to combinatorics.h and add test
  coverage.

Change-Id: Idf6685375ae16ee27c92a574eff96813c2091b81
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4853875
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Commit-Queue: Andrew Paseltiner <apaseltiner@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1195351}