SystemUI: screenshot: add extra crop boundaries

Add corner boundaries that allow resizing in both vertical and
horizontal directions at the same time.

To handle corner boundaries, split every boundary into vertical
and horizontal boundaries, and handle both vertical and horizontal
changes for all boundaries. If the boundary is a purely vertical
(top, bottom) or purely horizontal (left, right) boundary, the
opposite direction boundary will be none, and the selection won't
be changed on that selection.

Also, add a middle boundary that allows moving the current selection.

To achieve this, treat the middle boundary as composed of top and
left boundaries, and when trying to change either the top or the
left boundary while the currently dragged boundary is the middle
one, reposition the opposite boundary too.

Change-Id: I6dd703733b3d161f5979d91a124350fb1a8d52ae