I've been testing with 100.6.0 build 2470, and the navigation is working. I switched to your production release of 100.6.0 release last week 8/22/2019 and with this release I'm getting an error
com.esri.arcgisruntime.ArcGISRuntimeException: Rerouting for the provided online route task does not supported.: Rerouting for the provided online route task does not supported.
I'm getting this error on this line of code
routeTracker.enableReroutingAsync(routeTask, routeParameters, RouteTracker.ReroutingStrategy.TO_NEXT_WAYPOINT, true).get();
Unfortunately we have had to disable this functionality for 100.6 when working against a service-based route task. You can enable rerouting for local network datasets. We hope to re-enable this at 100.7 for your own published route services but cannot promise that the World Routing Service will support this. Please DM me if you have specific requirements or questions.