Fixes https://github.com/flutter/flutter/issues/128749

The ScrollPositionAlignmentPolicy does not account for AxisDirection, which meant default focus traversal of reversed scrollables did not work. The policy doesn't know about the axis direction, so this is corrected in the ScrollPosition where all the information is available before calling on the viewport for the new target scroll offset.

This fixes that by flipping the policy (unless explicit) so that focus traversal works.