libvpx

Fix rounding in ARNR calculation

The rounding of the ARNR filter output prior to
normalization by the filter strength was incorrect
when strength = 0.

In this case 1 << (strength - 1) would not create the
required rounding of 0, rather it would outrange. This
patch fixes this issue.

Change-Id: I771809ba34d6052b17d34c870ea11ff67b418dab