Force consuming system bars if nav bar is shown forcibly

This makes the decor view consumes the navigation bar insets, so the app
content won't be overlapped with navigation bar.

Fix: 276506304
Fix: 280915087
Test: Open immersive apps in kids mode. Make sure the app content won't
      be obscured by the visible navigation bar.
Change-Id: Ica4339f4bf9a71fd524501e60a9f69fb53a0bb56