transaction on the policy shard, not notification shard

notification is unsharded, so it would always be on the default shard

Change-Id: I7c821ff9e8a6827708e680432ba24972a3c329ce
Reviewed-on: https://gerrit.instructure.com/39450
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>