Remove metric arrival event reset for multiple waypoints

[danesfeder]

Closes #1012

The off route listener was not being removed upon arrival because the metricArrivalListener was null. This was a regression from #943. Now that we use milestones, the milestone listener will only fire once and we don't need to worry about resetting a listener / duplicate events as a result.

Using the three waypoints from the ticket:

[Guardiola31337]

Looking good 🚀

[Guardiola31337]

Looking good 🚀

But Firebase doesn't think so 😂

FATAL EXCEPTION: main
Process: com.mapbox.services.android.navigation.testapp, PID: 17527
java.lang.NullPointerException: Attempt to invoke interface method 'void com.mapbox.services.android.navigation.v5.navigation.metrics.NavigationMetricListener.onArrival(com.mapbox.services.android.navigation.v5.routeprogress.RouteProgress)' on a null object reference
	at com.mapbox.services.android.navigation.v5.navigation.NavigationEventDispatcher.checkForArrivalEvent(NavigationEventDispatcher.java:175)
	at com.mapbox.services.android.navigation.v5.navigation.NavigationEventDispatcher.onMilestoneEvent(NavigationEventDispatcher.java:135)
	at com.mapbox.services.android.navigation.v5.navigation.NavigationService.onMilestoneTrigger(NavigationService.java:125)
	at com.mapbox.services.android.navigation.v5.navigation.NavigationEngine$1.run(NavigationEngine.java:93)
	at android.os.Handler.handleCallback(Handler.java:739)
	at android.os.Handler.dispatchMessage(Handler.java:95)
	at android.os.Looper.loop(Looper.java:135)
	at android.app.ActivityThread.main(ActivityThread.java:5254)
	at java.lang.reflect.Method.invoke(Native Method)
	at java.lang.reflect.Method.invoke(Method.java:372)
	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)