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.

Change-Id: I976ffae3a56d411b00026f86b16efa9743a23293