Restructure of activity masking code.

This commit restructures the mb activity masking code
to better facilitate experimentation using different metrics
etc. and also allows for adjustment of the zero bin either
for encode only or both the encode and mode selection
stages

It also uses information from the current frame rather than
the previous frame and the default strength has been
reduced.

Change-Id: Id39b19eace37574dc429f25aae810c203709629b