make quiz stats use kept score instead of always using latest score

fixes CNVS-13561

test plan:
  - enable draft state and quiz stats
  - as a teacher
    - create a quiz that allows multiple attempts, and keep highest score
    - publish the quiz

  - as a student
    - take the quiz a first time, and get a perfect score
    - take the quiz a second time and get a worse score

  - as a teacher
    - view the quiz stats
    - it should show results from the kept score, not the latest

  - try this with keeping the highest score as well. Stats should use the
    correct kept score.

Change-Id: I8fb9998e800bfd91f9297a3bc30178f5f31aa59c
Reviewed-on: https://gerrit.instructure.com/36300
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>