Modified ARNR MC-filter to ignore ARF frame

The ARNR filter uses MC to find the best match between the
ARF and other nearby frames in the filter-set. Since the
ARF is a member of the filter-set, MC in that case is
unnecesssary. This patch modifies the filter so it does
not apply MC in this case.

Change-Id: Ic0321199c08db2189a57f28d1700b745bc7ff66d