Check whether channel has been removed before sending cancelation.

When a gesture monitor pilfers pointers we synthesize cancelation
events for all currently active windows. It's possible, however, that a
window may have been removed between the beginning of the touch stream
and now, so we have to check whether the corresponding channel still
exists.

Bug: 145891806
Test: atest InputDispatcher_test.cpp
Change-Id: If16191b02bbb35886171c402b9bb74f547bdf94a