Remove over aggressive optimization.

It turns out that sometimes the wallpaper target is migrated to the
bottom of the window stack and then mWallpaperTarget is set to null.
In particular this happens when the launcher all-apps screen is
brought up. When this happens the layer of the wallpaper is
correctly set below the previous wallpaper target.

An optimization in WindowAnimator was keeping the layer update from
propagating to the Surface object. This fix removes that optimization.

Fixes bug 6631717.

Change-Id: I800dd043ce8df83b4e5edbf710503135396bc01e