DisplayUtilityAssociations: AddAssociations is called twice during initialization

[mbranscomb]

DisplayUtilityAssociations: AddAssociations is called twice during initialization

AddAssociations is called once in private async Task Initialize and then again in private void OnNavigationCompleted:

https://github.com/Esri/arcgis-runtime-samples-dotnet/blob/b6d103e48c8992769e999a84d7af93489c21ffc6/src/WPF/ArcGISRuntime.WPF.Viewer/Samples/UtilityNetwork/DisplayUtilityAssociations/DisplayUtilityAssociations.xaml.cs#L135

https://github.com/Esri/arcgis-runtime-samples-dotnet/blob/b6d103e48c8992769e999a84d7af93489c21ffc6/src/WPF/ArcGISRuntime.WPF.Viewer/Samples/UtilityNetwork/DisplayUtilityAssociations/DisplayUtilityAssociations.xaml.cs#L145

[mbranscomb]

@ZackAllen @duffh