query the account's shard for enrollments in account policy block

refs CNVS-11420

Change-Id: Iedd7d6b97b19fa636d6cdd536ca7e5914ef12743
Reviewed-on: https://gerrit.instructure.com/31001
Reviewed-by: Rob Orton <rob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>