Fix issue #7353900: Brief flash of home screen while dream starts on lock screen.

Not very clean, this has a special hack in the window manager to
redo layout when a dream window is shown.  After MR1 we should clean
this up (and the various other special dream hacks).

Change-Id: Ic1a5a2b10a0a07b4a5dccdbf0736b614ec06dd4a