Update pre-drag lifecycle for apps with shortcuts.

- First of all, deferred drag has been renamed to pre-drag
  to avoid confusion with the existing deferred end drag.
- For normal drags, the cycle is still startDrag -->
  onDragStart --> onDrop --> onDropComplete --> onDragEnd.
- Pre-drags have two additional callbacks: onPreDragStart
  and onPreDragEnd. onPreDragStart is called between
  startDrag and onDragStart, and onPreDragEnd is called
  at the same time as onDragStart or onDragEnd.
- If the pre-drag has not transitioned to a full drag before
  onDragEnd, onDragStart and onDropComplete are skipped
  (onDrop is still called to allow the DragView to animate).

Change-Id: Icd7a8f75d5fcc159f9a52758c22ab6eae3edb9e2