Coding the hybrid dual prediction signal.

Initial modifications to make limited use of common prediction
functions.

The only functional change thus far is that updates to the probabilities are
no longer "damped". This was a testing convenience but in fact seems to
help by a little over 0.1% over the derf set.

Change-Id: I8b82907d9d6b6a4a075728b60b31ce93392a5f2e