Bind to the correct face lock service.

When switching users, bind to the correct instance of FaceLock.

Tried to get the facelock screen to show up when switching users, but it
doesn't. Power off/on works, but not switching between users on the
lockscreen. Maybe a timing issue? Or not calling some function to make
the overlay visible?

Change-Id: I9300971331c7f0ed93ae7da28170342629426b20