Make the Pholio Mock "getImages" / "getAllImages" API more clear

Summary:
Depends on D19920. Ref T11351. Currently, "images" and "all images" are attached to Mocks separately, and getImages() gets you only some images.

Clean this up slightly:

• One attach method; attach everything.
• Two getters, one for "images" (returns all images); one for "active images" (returns active images).

Test Plan: Browsed around Pholio without any apparent behavioral changes.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T11351

Differential Revision: https://secure.phabricator.com/D19921