Animate icons back to where they were picked up.

We do this for pre-dragged icons instead of calling onDrop().
- Removes need for special logic in onDrop() to check if mIsInPreDrag
- Dropping from pre-drag in all apps also animates instead of jumping

We also do this when dropping an icon while still transitioning
to spring-loaded mode, to avoid having a janky jump at the end
- Bug: 27135377

Change-Id: I4548c3e2fef3423d1ba36057fb53807b1b4ad0fc