Rendering fixes for assist handles.

Compute the arc angle based upon a fixed distance along the perimeter.
If that angle exceeds 90 degrees, add "arms" extending from the 90
degree corner and lighten the stroke slightly.

Fixes corner display on small-radius phones.

Bug: 134487710
Fixes: 133834204
Test: Visual inspection with UX across several devices.

Change-Id: I6ce625d14bd7224114da311e54c86b87e92533a1