Don't exit immersive mode while showing a floating window

While a floating window becomes the focused window, status bar would
stay hidden if the top fullscreen app hides it. This is the existing
logic. This CL applies the same logic to navigation bar.

Fix: 225684651
Test: 1. Open Window Insets Test > Window Insets Controller.
      2. Hide navigation bar.
      3. Click on the drop-down menu.
      See if navigation bar stay hidden.
Change-Id: I45ec5ec3312ea98f1c65886e509d2b3029603df7