There was a concious decision that when we add a new wallpaper we should
append to the existing random order collection, rather than shuffle
existing ones.
The logic is fine but if we start with 0 wallpapers and then add 20 new
ones we don't shuffle anything.
This patch changes it so when a new batch of N wallpapers is added,
those N are shuffled when appending to the existing sort order.
BUG: 413463
FIXED-IN: 5.17.2