Fixing crash upon cancellation of an accessible drag.

This fixes perhaps an old bug.

If we started an accessible drag for an only item on a page,
then uninstalled the app while dragging, the page was removed
without unsetting its accessibility delegate. Later, the system asks
the delegate to do something, but the drag is over, and some pointers
are null, so everything crashes.

Fixing this.

Bug: 22028725
Change-Id: I85adcd42ae896603634994e20a7790792f7e84b1