Fail more gracefully when window state changes during drags

In particular, if a new window was created out from under the drag,
the drag-move event dispatch logic would wind up blithly assuming
that it would find a valid touch-dispatchable window under the
finger.  When this was untrue, it would NPE, but the NPE was caught
and discarded safely by higher-level guard code.

With this change, we now avoid the NPE entirely and report the
spurious condition as part of the debugging log stream.

(Also add the local-state object to DragEvent.toString()'s output.)

Change-Id: Idfa5f45d049e48415ee59c8b0ffb5b0ed4098ce3