Check dispatcher on announcement and instruction events in ViewModel

[danesfeder]

Found in CI:

Fatal Exception: java.lang.NullPointerException
Attempt to invoke virtual method 'com.mapbox.services.android.navigation.ui.v5.voice.SpeechAnnouncement com.mapbox.services.android.navigation.ui.v5.NavigationViewEventDispatcher.onAnnouncement(com.mapbox.services.android.navigation.ui.v5.voice.SpeechAnnouncement)' on a null object reference
com.mapbox.services.android.navigation.ui.v5.NavigationViewModel.playVoiceAnnouncement (NavigationViewModel.java:368)
com.mapbox.services.android.navigation.ui.v5.NavigationViewModel.access$700 (NavigationViewModel.java:47)
com.mapbox.services.android.navigation.ui.v5.NavigationViewModel$3.onMilestoneEvent (NavigationViewModel.java:282)
com.mapbox.services.android.navigation.v5.navigation.NavigationEventDispatcher.onMilestoneEvent (NavigationEventDispatcher.java:137)
com.mapbox.services.android.navigation.v5.navigation.RouteProcessorThreadListener.onMilestoneTrigger (RouteProcessorThreadListener.java:46)
com.mapbox.services.android.navigation.v5.navigation.RouteProcessorHandlerCallback$1.run (RouteProcessorHandlerCallback.java:98)

These NavigationViewDispatcherEvents were added in #1107 and were not accompanied with a null check. Every time we use the dispatcher in NavigationViewModel we need to double check it isn't null because we set it to null during rotation to prevent leaks from the NavigationView being destroyed.

[Guardiola31337]

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