Improve implementation of ManiphestTaskQuery

Summary:
Currently, we have a single projectPHIDs field, and a separate flag which makes it act like AND or OR.

This is silly. Make two separate methods for setting AND vs OR projects. This also simplifies the implmentation.

This doesn't change the UI or any behavior (yet), it just makes the API more usable.

Test Plan: Loaded homepage, "All Projects" task view, verified queries made sense and returned correct results. Grepped for changed method name.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1610

Differential Revision: https://secure.phabricator.com/D3630