Dragging an icon out of a folder was showing a gray background. This was due to the icon's drawing cache background color. This fix simply temporarily resets that color when capturing the view into a Bitmap.
This change adds nearby-area searching when dragging desktop items into already-occupied cells. This approach tries harder to find a matching area, instead of strictly rejecting invalid moves.
We also draw a "snag" during the drag to show where an item would be dropped, but only if we would drop into an alternative cell. This gives users better feedback about where things will drop.
http://b/issue?id=1634887
