Ignore attempts to finish events on unregistered channels.

This is a common race that happens during application shutdown where the window
may be removed before the input event is finished.  The input dispatcher
already recovers from this condition gracefully so there are no benefits to
throwing an exception on the client side.

Bug: 2834068

Change-Id: I53dcc3230464d7f528ac8a1cc9f01b5bb642f428