Defaulting wallpaper offset to edge of the screen.

- To match the AOSP launcher which no longer centers the default page, 
  we now align the wallpaper to either edge of the screen (depending on 
  RTL).  This prevents a jump in the keyguard when starting the device
  due to the wallpaper offset being initialized before Launcher is 
  started and can update the offset itself.

Bug: 28795125
Change-Id: Ica96a367157aef696b361601d346774960430fbf