Make sure system bars have visible background while force-showing

Client insets controller can request to hide system bars to enter
immersive mode. However, there're cases force-show system bars like in
split screen or freeform mode. This makes sure the background view of
system bars are always visible while force-showing.

Fix: 234196894
Test: put an immersive app in split, verified system bars background
      won't be invisible
Change-Id: Ia167bce9e64cd81a2ade499f8c6528384d957209