Remove NavigationViewModel as lifecycle observer

[danesfeder]

Closes #623

When using a Fragment implementation of NavigationView the NavigationViewModel will still use the Activity Context when adding NavigationViewModel as a lifecycle observer.

This means that the view model isn't properly shutdown if the Fragment is hidden and not destroyed (and the Activity is still running).

This PR removes the NavigationViewModel as a LifecycleObserver and moves the lifecycle method onDestroy to NavigationView#onDestroy, allowing the developer to call this whenever they chose (like Fragment#onDestroyView).