Correct behaviour of update function to handle multiple wallpapers

Fix: 285843200
Test: manual: open secure camera, take a picture and open it without
unlocking; verify that when going back to lockscreen, the home wallpaper
is never visible
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:a4eb5d06541c9de432346fb96a58e725a184fb78)

Change-Id: Ibe199a6678a27b4c8cbd38cb8085c9296e0a6e13