Have wallpaper target itself when dismissing keyguard with wallpaper

We were previous setting the top app window as the wallpaper target
when the keygaurd is animating away with the wallpaper. This causes
a window that doesn't have the flag to show wallpaper behind it to
become the wallpaper target and it remains the target as long as
there are no window movements. Instead we can make the wallpaper
window target itself during this transition.

Bug: 29092234
Change-Id: If4e3972e7012c6605b1f9dfb9234ceda8db8738b