Simplify value decoding for PHID custom fields

Summary:
Ref T9123. The handling in D14183 didn't deal with new field values properly.

Make all this handling more consistent.

Test Plan: Created a new WorkignCopy build plan with some repos.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9123

Differential Revision: https://secure.phabricator.com/D14184