Use better conditional probability for ngram entries.

Old:
P(W | W_prev) = f(W, W_prev) + C
New:
P(W | W_prev) = f(W, W_prev) / f(W_prev)

Bug: 14425059
Bug: 16547409

Change-Id: I4d13be6de2c6bad6bad7fb22320a23ba4ecd361c
2 files changed