TFTS: Move normalization to the base class, start using it for state space models

Preivously, state space models adjusted their priors based on the data
(e.g. setting initial variances to match sample variance) but did not normalize
the data itself. When the data has a rather extreme scale, this runs into
precision issues. After this CL, state space models will first normalize, then
use adjusted statistics on top of that normalization to estimate initial
observation/transition noise.

Also fixes an issue where start-of-series statistics were incorrect for the first
batch (which only shows up with large input scales).

PiperOrigin-RevId: 171044863