getActivityTokenBelow should exclude finishing activity

This method was introduced in ag/15302841 to check the new launch
activity and the activity below.

However, when launching an activity into split, the organizer may also
call to finish existing activity in split. We don't want to check the
split rule with a finishing activity.

Bug: 190433398
Test: test with demo app
Change-Id: I7f604ddea7c2f68adc3cee52b8872e653d5dad98