title: Use default back button behavior with android navigation component
body: <p>I am using a navigation graph in my app. I have a bottom navigation bar. I am staring my navigation components with the following in my Main Activity:</p>
  <pre><code>setContentView(R.layout.activity_main)
  val navView: BottomNavigationView = findViewById(R.id.nav_view)
  
  val navController = findNavController(R.id.nav_host_fragment)
  // Passing each menu ID as a set of Ids because each
  // menu should be considered as top level destinations.
  val appBarConfiguration = AppBarConfiguration(setOf(
          R.id.navigation_artist_list, R.id.navigation_dashboard, R.id.navigation_notifications))
  setupActionBarWithNavController(navController, appBarConfiguration)
  navView.setupWithNavController(navController)
  </code></pre>
  <p>This starts my navigation components, which inflate the fragement setup in <code>R.id.navigation_artist_list</code>.</p>
  <p>From an observer in that fragment I am navigating to another fragment with:</p>
  <pre><code>viewModel.selectedAlbum.observe(this, Observer { artist -&gt;
      val action = HomeFragmentDirections.actionNavigationHomeToAlbumFragment(artist)
      root.findNavController().navigate(action)
  })
  </code></pre>
  <p>However, none of the mechanisms available for navigation work properly. The back button just refreshes the fragment and populates the list in the fragment again. The back button on the action bar is completely ignored.</p>
  <p>Do I need to do something else to have the proper back button behavior, to go back to the previous fragment?</p>
  <p><a href="https://i.stack.imgur.com/lNbXA.gif" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/lNbXA.gif" alt="video of problem" /></a></p>