Change the formula of the missing character.

- Bug: 4271049
- Due to the result of the recent user study, a word with a missing character needs to be promoted a bit.

so I changed the formula from:
- freq * 70 * (n - 2) / (n - 1)
to:
- freq * 90 * (10n - 12) / (10n - 2)

Change-Id: Ibff72cbdb0f2d7b91460a06a0fd39a9f5749aa46