Account for wallpaper scale in prepareSurfaceLocked

This fixes a flicker in the wallpaper when rotating the
screen that was caused because prepareSurface was scaling
and translating the surface without considering the zoom-in
scale that is applied everywhere else to wallpapers so the
end result was the surface being scaled down then up again.

Also clean up a bit the usages of setWallpaperPositionAndScale

Bug: 152002715
Fixes: 152376185
Test: manual
Change-Id: I2d2d74a411bee13bee1a7b4cce3744ee450ac549