Enable lazy-loading of security views in keyguard

This change removes all but the default KeyguardSelectorView from KeyguardViewHost
to (1) reduce the overall memory footprint (2) reduce initial layout inflation
time and (3) avoid initializing the camera until needed by face unlock.

Fixes bug 7127666

Change-Id: Ibac1838dd7a490dcadbfab5bdfdd82734b69055a