Reinforce user-history based bigram use.

- Have user-history based bigrams used as soon as they are entered,
  not after 6 times.
- Limit bigram frequency to 255 (this limits the multiplier to 1.8, and
  has no effect on main dict bigrams which already have this limit)
- Some TODO resolving

bug: 4192129

Change-Id: I4777f1a58c43dd55381c4b01252d722ab3a70547