Re-factor and simplify arnr filter.

Use a weaker filter for second level arf frames.
Average gain across all sets and metrics ~0.3%

Remove code for arnr_type which is no longer
supported in VP9 which always uses a centered blur.

Re-factor and some cleanup.

Change-Id: Ieb4b8940e99e4e02b3fcc9fca6f2d4109e6ed639