Allow touch split enabled when no window touched

If no window can be touched by first touch down, it would still deliver
the event to the gesture monitor. So before gesture monitor pilfer the
touch, if a new window supports split touch, it can't recevied the new
point down event cause device looks unresponsive.

Test: atest inputflinger_test
Bug: 201647070
Change-Id: Ib0734ef7082bf673afb11eef9ec7bdb0b3f103ec