Fix cross-app drag and drop crash.

A DragEvent created in DragState.broadcastDragEndedLw
could be recycled twice if one of the notified windows
belongs to the system process. In this case the same
instance of DragEvent is recycled first inside the event
handler, and then the second time before exiting the
function (which causes a RuntimeException).

Added a Pid check similar to another four places in this
class where dispatchDragEvent is called.

Bug: 25079983
Change-Id: Ia7c1b9258a0c624bc7e2451650e2fb362848a16d