Restore jankless transition keyguard-to-wallpaper

There is still a flash of black when going to a non-wallpaper activity
from keyguard. This is not a regression from jb-mr2 and any fixes to
clean it up are too risky at this late date.

Fixes (partially) bug 11570753.

Change-Id: I17aaae4ab8be570f7e28276a7b8ac4b8685e7551