Fix NPE in InputDispacher by enabling debug logs

InputDispatcher will manage input channels and input windows.
Once some input channel unregistered, there is a chance to
find the corresponded target before setInputWindow.
Then input channel of input target would access the null pointer.

Bug: 120307496
Test: Enable debug log, run monkey test serveral times.
Test: atest inputflinger_tests
Change-Id: Ia7b36e0732e0ac6c825f3910fd02b1cdbbf85161