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