Fix up expanded desktop behaviour.

The expanded desktop mode had a number of issues:
- It didn't handle multi-user scenarios correctly
- In the keyguard, the status bar was shown while the navigation bar was
  hidden, which was inconsistent to the !keyguard case
- Live wallpaper sizing was incorrect when expanded desktop mode was
  enabled due wallpaper windows not being recalculated correctly after
  enabling expanded desktop

This patch should fix all of those items.