Refinements to drag/drop

Thumbnail measurement & drawing has been moved out into a delegate
class called View.DragThumbnailBuilder.  This consolidates the
thumbnail-related code as well as ensuring that the drag initiator
does not have to know a priori where to place the thumbnail relative
to the touch point *before* the thumbnail measurement step, as was
previously the case.

startDrag() no longer needs to be told where the current touch point
is at the time the drag is launched.

Drag events are now dispatched only to VISIBLE views.

Dispatch shouldn't double-recycle events any more when the target
window is local to the system process.

Change-Id: I49419103765a0cad2e18ddfcdd6dacb94daf1ff1