Check for mSurfaceController in setWallpaperPositionAndScale

 WindowSurfacePlacer#performSurfacePlacement calls WallpaperController#updateWallpaperOffset
 before the wallpaper window creates its surface control

Test: adb reboot;
 adb shell logcat | grep 'Attempt to invoke virtual method 'void com.android.server.wm.WindowSurfaceController.setPositionInTransaction(float, float, boolean)' on a null object reference'
Bug: 155998607
Change-Id: Ib1483a5849192b1a42c14383e6dd9e96684de2cc