Display generic icon for roundabout instruction without heading

[1ec5]

OSRM-based profiles of the Directions API apparently leave the degrees property of a banner instruction object unset if the leg ends on the roundabout (avoiding an exit roundabout maneuver). When this happens, we should display a slight right turn arrow (slight left turn in left-side-driving countries) to suggest entering a roundabout without saying exactly how far to go within the roundabout.

This heuristic is unnecessary for Valhalla-based profiles, which provide a heading. It also doesn’t apply to routes that exit the roundabout; in that case, all profiles provide a heading.

/ref mapbox/mapbox-navigation-android#3867 /cc @mapbox/navigation-ios @Guardiola31337 @pjleonard37