Fix dragging between icons in the app shelf

Convert the icon margin into internal padding. This ensures
that DRAG_ENTERED events are created when the icon is visually
dragged between the icons.

Also change the drag shadow size computation to force it to be
square, because the icon ImageView is no longer square.

Bug: 22603558
Change-Id: I469df043448e562269b764245135fbf898f004ea