cros: Use first unlock user's pref for locking screen.

Use first unlock user (which is the primary user at the
moment) pref for locking screen;

BUG=322662

Review URL: https://codereview.chromium.org/83543004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236931 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236931