Reduce floating point error in PerimeterPathGuide.

Instead of dividing before adding up the lengths, add up the lengths and
then divide.

Before this change, the end point of the last region was not 1.0, after
this change it is.

Bug: 136084565
Test: Verified values via logging. No visible change in behavior.
Change-Id: Iddc87bed3c8b427af41f247589a9f1a99ad03555