improved a problem that continues to maintain a touch-ID that must be discarded by 'touchesCanceled' event. Fixes golang/go#13435 Change-Id: I13635255e5317bba2202bf7a8d2dd957f7824d0a Reviewed-on: https://go-review.googlesource.com/18180 Reviewed-by: David Crawshaw <crawshaw@golang.org>