Reset fingers and buttons in GestureConverter::reset

Previously, calling reset didn't really reset anything, which could have
resulted in an inconsistent event stream (e.g. fake fingers that were
put down but never raised).

Bug: 251196347
Bug: 259547750
Test: atest inputflinger_tests
Test: scroll, swipe, and pinch on a touchpad, check dispatched events
Change-Id: If95cde96f1974bf7c98009c68e2637686bff96cc