SBA DirAC 20ms parameter estimation at LBR bug fix

We stumbled across a bug in encoder side SBA DirAC parameter estimation for LBR (12.3/16.4kbps) modes. It appears that the code was not iterating over 5ms subframes correctly, and instead processed the first subframe four times. The proposed fix iterates over all four sub-frames.

We have done analysis with PEAQ, and some informal internal listening, and observed slight improvements in some vectors, and no degradations otherwise.