Hide taskbar background when notification shade is expanded

- In 3 button mode, don't hide the background but do ensure nav buttons are translated down when swiping to notificaitons from the home screen

Test: swipe down notifications, background hides in 0 button mode but not in 3 button mode
Fixes: 199163951
Change-Id: Ic70adc7d67b2ab7c92a2d5d846b9b259d1bd7bfc