Set parallax offset to be 0.5f for default wallpapers.

This ensures they are centered as they were without any offset.

Bug: 25347076
Change-Id: Id773cd89463d47b350e60878c00e1112e0244414

WallpaperPicker/src/com/android/launcher3/WallpaperPickerActivity.java

@@ -309,7 +309,7 @@ public class WallpaperPickerActivity extends WallpaperCropActivity {
 
                 @Override
                 public float getParallaxOffset() {
-                    return 0;
+                    return 0.5f;
                 }
             };
             req.result = new DrawableTileSource(a.getContext(),