Make PointerEventDispatcherTest faster

Before this CL, the test waited 10 seconds (on hwasan builds) to assert
that there aren't any events.

Make this check faster by adding a separate assertion that events didn't
occur.

Bug: 290931418
Test: atest PointerEventDispatcherTest --rerun-until-failure 1000

Change-Id: Id4582ab0a2b58787a453e06250b82ecf2b1720f0