Ensure WallpaperEngine has correct size information

Currently it's possible for the WallpaperManagerService to receive a
request to change the desired wallpaper size between when it attaches
to the WallpaperService and when the Engine attaches back to it. This
means that the Engine will be created with the prior dimensions and
won't receive the update. Check to see if we're in this state and
make note to update the Engine once it attaches to ensure that the
Engine always has the correct dimensions.

Bug: 10853302
Change-Id: I8d5cd3371b269dbb4c0ff2d75c68529b69028ae5