title: Shared ViewModel's onCleared() is never called
body: <p>I have a single Activity application. In one part of the app there is a master-detail combination of Fragments that use <a href="https://developer.android.com/topic/libraries/architecture/viewmodel#sharing" rel="noreferrer">a shared ViewModel</a> to share some data between them. The problem I face is that the ViewModel's onCleared() method is not called even when both of the Fragments have been destroyed. onCleared() is only called when the activity is destroyed. </p>
  
  <p>Is this how it is supposed to work? Because that is quite useless in a single Activity model, because the Activity is always alive. Or am I missing something?</p>