Show "Add users from lock screen" dependent on secure lock screen.

SysUI only enforces this setting while a secure lock screen is set,
so it doesn't make sense to show it configurable while an insecure
lock screen is set.

Bug: 148015798
Test: make -j64 RunSettingsRoboTests ROBOTEST_FILTER="com.android.settings.users.AddUserWhenLockedPreferenceControllerTest"
Change-Id: I186b2339e9575e20b55e2e5c1dab3de624848fc5