Don't inappropriately kill ANRing drop recipients

If an app takes the 5-second ANR timeout before responding to a
drop, but then recovers, we were inappropriately throwing an
exception back at it for having acknowledged the drop after we'd
abandoned the operation out from under it.  Now we let such
responses slide without taking any punitive action: the app is
still okay, and the drag/drop operation was cleanly terminated
already anyway.

Bug 5045618

Change-Id: I0b7e76c61f0f8c97e41280b542a470a7d3c8d86f