InputDispatcher: Fix crash when there is an ANR after window removal

This addresses a bug in the change:
I47744cbd677cc74e26a102c50a2c11c68bc8aa89

InputDispatcher has an invariant that it can only send events to a
connection if it has a window. We did not check if the channel receiving
an ANR had a window before attempting to synthesize cancellations for
it.

Bug: 324330557
Bug: 210460522
Test: atest inputflinger_tests
Change-Id: I5f3013fe93c0f4d1bb0f58e7b2a241cffe5c5bf2