Refactoring FlingToDelete

> Moving all fling related logic to FlingToDeleteHelper from DragController
> Removing fling related methods from DragSource and DropTarget
> Moving fling animation logic from DeleteDropTarget to FlingAnimation
> Simplifying DropTargetBar to directly look for all valid drop targets.
  This makes it easier to add new DropTarget in xml.

Change-Id: I7214d2d30c907ab93c80d92d9f9be6dda2d63354