Convert file status to enum

Use an enum to store the file status in FileInfo
GerritControllerActivity: Added some null checking
PatchSetChangesCard: Removed some dead code

Change-Id: I11492f7c5622c910dbc5a62540254e1ed3d1f79b
JSONCommit: Convert reviewers getters to List

Change-Id: I71e54136a4b4800043d54be729f52e80c490a497
ReviewerList: Add method to get all labels reviewers are associated with