Fix bug in calculating path in ArcMotion

Bug: 73077523

When the path is perfectly horizontal, ArcMotion
was calculating a Path that had NaN values. This
corrects that problem by special casing horizontal
and vertical paths.

Test: manual with app that discovered the problem
Test: manual with test app
Test: I30d51206194e3c68ea145d3a81e05a461c4e0ca8
Change-Id: Ic1a70b79290847726fc7994d1224fd77024e0610