Some minor fixes for extracted status bar.

- setLightStatusBar() is now updateStatusBar(), with a forceLight
  parameter. We set the status bar to be light if forceLight or
  shouldBeLightStatusBar() (based on wallpaper).
- Force status bar to be light if all apps is open.
- Default to dark status bar, not light (light == dark icons)

Bug: 29452834
Change-Id: I7b102ceff2f1ef2ab8defd4a46c698df4feaf2a5