Allow dragging navbar app icons from recents to pinned

* Use an activity launch Intent as drag data for both pinned apps and
  recent apps in the shelf
* Create an invisible placeholder app icon when dragging into the pinned
  app area
* Use the drag listener on the app icons only for drag-entered; use the
  entire NavigationBarApps ViewGroup for start/drop/end. This allows
  centralized setup and cleanup of drags.
* Extract IconDragShadowBuilder into a shared class

Bug: 20024603
Change-Id: Iaa698500f0d66f91220e7fac0eea5115181a0156