Adding lock icon to FaceLockAreaView on lockscreen fixes 5394522

This makes the transition from the backup to Face Unlock invisible to the user,
using three layers:
the base background layer (also used in Face Unlock)
the spotlight mask layer (black alpha = .8), representing a size-0 spotlight
The cancel button, which would incorrectly resize to full-screen if a View with
a background were used instead of the current ImageView

Change-Id: Ieb433baf171088e0af06f183fcdb5ddda6001f01