Voice instruction milestone crash

[Guardiola31337]

While working on #697 found 👇

04-06 20:55:13.803 25521-25704/? E/AndroidRuntime: FATAL EXCEPTION: NavThread
                                                   Process: com.mapbox.services.android.navigation.testapp, PID: 25521
                                                   java.lang.NullPointerException: Attempt to invoke virtual method 'void com.mapbox.services.android.navigation.v5.navigation.VoiceInstructionLoader.cacheInstructions(com.mapbox.services.android.navigation.v5.routeprogress.RouteProgress, boolean)' on a null object reference
                                                       at com.mapbox.services.android.navigation.v5.milestone.VoiceInstructionMilestone.isOccurring(VoiceInstructionMilestone.java:30)
                                                       at com.mapbox.services.android.navigation.v5.navigation.NavigationHelper.checkMilestones(NavigationHelper.java:201)
                                                       at com.mapbox.services.android.navigation.v5.navigation.NavigationEngine.handleRequest(NavigationEngine.java:76)
                                                       at com.mapbox.services.android.navigation.v5.navigation.NavigationEngine.handleMessage(NavigationEngine.java:41)
                                                       at android.os.Handler.dispatchMessage(Handler.java:98)
                                                       at android.os.Looper.loop(Looper.java:148)
                                                       at android.os.HandlerThread.run(HandlerThread.java:61)

Steps to trigger behavior

1. Navigation test app (dan-maps-3.0 branch)
2. Open Mock Navigation activity
3. Tap map to place waypoint
4. Press the START ROUTE button
5. 💥

It seems that VoiceInstructionLoader from VoiceInstructionMilestone is not properly initialized because if you launch a navigation session from the Navigation Views activity and then repeat above steps it works fine. Noting that VoiceInstructionLoader is a singleton and Mock Navigation activity doesn't reproduce voice instructions.

👀 @devotaaabel @danesfeder