Show the live wallpaper on the lock screen.

This also takes care of the problem of system dialogs like the
crash dialog causing the status bar to dim behind the lock screen.

On the down side, the fade transition from the lock screen is
now gone, and I'm not sure how likely it is for it to return.

Change-Id: I7f9e6d0f3510a1fdbbe6ad252d986bd85a16475d