Added default config to enable adding users from lockscreen

A config value is added for enabling/disabling the ability
to add users while screen is locked. The config value, currently false,
is read and applied to the setting only if the user has not set the
value as true/false explicitly.

Bug-Id: b/24662310
Change-Id: Ie00ab0952c64cbc9805794bc0dde350920750026