title: how not to destroy fragment with androidX
body: <p>now ,I try androidX's navigation and bottom-navigationbar,when I use it like below 
  <code>
  supportFragmentManager = getSupportFragmentManager();
  navHostFragment =(NavHostFragment)supportFragmentManager.findFragmentById(R.id.frag_nav);
  navController = navHostFragment.getNavController();
  NavigationUI.setupWithNavController(navigation, navController);</code></p>
  
  <p>I found an issue,everytime switch the bottomNavigationBar,the fragment will be recreate,all network task in target fragment will be redo,how to keep fragment's state when it switch in androidX?</p>