Fix 5044158: Reduce overall memory footprint of lockscreen

On devices that do not require showing lock before unlock, we now only create
just the unlock screen.  In addition to simplifying the code, this also
prevents keeping around a potentially unusued lock screen and saves ~1MB
in the system process on devices with only a secure lockscreen.

Change-Id: I533f2692b44a7991d4850cecc874c76166e7ce71