HY-841: Fix KeyNavigator Snap to Top Bug

**Summary**
When KeyNavigator responded to a horizontal arrow press followed by a vertical arrow press, it snapped the scrollList back to the top. This can be recreated on current wf-uicomponents master.

**Solution**
The snapping to the top was caused by calling ScrollToPosition with negative values for the `y` option. This is now being prevented by validating y values before use.

**Unit testing**
A unit test has been added to check for the behavior.

**CR**
Build this branch and enter the ScrollList Demo. You should be able to press a horizontal arrow key followed by a vertical arrow key and not be relocated to the top of the ScrollList.

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Please review: @patkujawa-wf @ryanhanks-wf 
FYI: @timmccall-wf