Allow disabling the lock screen when multiple users are present

Disallowing disabling the lock screen when multiple switchable users
are present on the device was a legacy from the time when the lock
screen was the only way of switching between users. As we offer other
ways to switch users now, e.g. quick settings, we no longer need this
check.

Also introduce a per user version of disabling the lock screen.

Bug: 19962043
Change-Id: I131568a5dadd1706762b8a626e8f9b06e973b7ae