R1:28989ac23145

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…