Add simple version of activity masking.

Add simple version of activity masking.

This uses MB variance to change the RDO weight for mode decision
 and quantization.
Activity is normalized against the average for the frame, which is
 currently tracked using feed-forward statistics.
This could also be used to adjust the quantizer for the entire
 frame, but that requires more extensive rate control changes.
This does not yet attempt to adapt the quantizer within the frame,
 but the signaling cost means that will likely only be useful at
 very high rates.

Change-Id: I26cd7c755cac3ff33cfe0688b1da50b2b87b9c93