Use the configured viewer more consistently in the Herald commit adapter

Summary: See PHI276. Ref T13048. The fix in D18933 got one callsite, but missed the one in the callConduit() method, so the issue isn't fully fixed in production. Convert this adapter to use a real viewer (if one is available) more thoroughly.

Test Plan: Ran rules in test console, saw field values. Will test in production again.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13048

Differential Revision: https://secure.phabricator.com/D18950