Modernize FeedQuery a little bit

Summary: Ref T12762. Updates some conventions and methods. This has no (meaningful) behavioral changes.

Test Plan:

• Grepped for setFilterPHIDs().
• Viewed main feed, user feed, project feed.
• Called feed.query.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12762

Differential Revision: https://secure.phabricator.com/D18027