Remove duplicate polyline decoding

[ericrwolfe]

Polylines are decoded 6 times on every new location update (9 times if a new step!) in NavigationEngine.handleRequest:

mapbox-navigation-android/navigation/libandroid-navigation/src/main/java/com/mapbox/services/android/navigation/v5/navigation/NavigationEngine.java

Lines 63 to 72 in 05a2f56

	private void handleRequest(final NewLocationModel newLocationModel) {
	final RouteProgress routeProgress = generateNewRouteProgress(
	newLocationModel.mapboxNavigation(), newLocationModel.location());
	final List<Milestone> milestones = checkMilestones(
	previousRouteProgress, routeProgress, newLocationModel.mapboxNavigation());
	final boolean userOffRoute = isUserOffRoute(newLocationModel, routeProgress);
	final Location location = !userOffRoute && newLocationModel.mapboxNavigation().options().snapToRoute()
	? getSnappedLocation(newLocationModel.mapboxNavigation(), newLocationModel.location(), routeProgress)
	: newLocationModel.location();

• 3-6x in generateNewRouteProgress():
  • 1x in userSnappedToRoutePosition()
  • 2x in stepDistanceRemaining() first here and also here via nextManeuverPosition()
  • Repeat all of the above if it's a new step
• 1x in isUserOffRoute() via userTrueDistanceFromRoute()
• 2x in SnapToRoute.getSnappedLocation() via snapLocationLatLng() and snapLocationBearing()

Polyline decoding is expensive especially on long route steps and if GPS updates are frequent, so decoding the polylines a single time when you parse the route json and storing the resulting List<Position> coordinates as part of each LegStep will help significantly with memory usage and general performance.

cc @cammace @danesfeder