Address race condition with drag from external window

When dragging from another window, e.g. when pinning an app shortcut,
there's a race condition as follows if launcher is killed:
- System drag starts
- Workspace loads and starts binding, canceling the drag

With this change, the drag will get onDragStart() again after this
case and allow completion of the drop.

Change-Id: I3f1c786ad1d451ecda9bbac5e9a9a3e0d8b66f73