Consolidating the logic for which API request to make in order to get a
fully populated file list into the API interface simplifies things.
Making UI components like the file list patchset-agnostic follows a
pattern set in previous edit related changes and centralizes fetching
logic.
Also removes some spurious/redundant tests.
Bug: Issue 4437
Change-Id: I21361ef3af8ab50c085248292b43bd4f8325afab