Add lock-to-app to settings

Lock-to-app when disabled shows some instructions about how to
enable it and use it.

When enabled shows an option of use lock screen, which controls
whether the device is locked when exiting lock-to-app.  Turning
on use lock screen requires that the device have some security
challenge.

Bug: 15759649
Change-Id: I6188243e03fb0c85bdfdbc32a23ad486296a34f9