Convert Diff Dialog into an activity

Add images for previous and next buttons
Bugs: Diff header text can be duplicated in diff view;
Spinner needs two different layouts and the non-drop down
 one needs to only show the short file name (can be elipsised)
TODO: Diff file navigation (buttons and spinner events)

Change-Id: I8563e594d530e005b35991fd4ef53d47a4f30ae5