Add affinity model for contact names.

This allows us to:
1. Rank contacts and only add the top N names to the keyboard LM.
2. Avoid adding duplicate names.

Note: The affinity calcualuation is limited by the fact that some apps
currently do not update the TIMES_CONTACTED counter. To better handle
this case, the new measure also takes into account whether or not a
name is in the visible contacts group.

Bug: 20053274
Change-Id: I2741cb8958667d4a294aba8c437a45cec4b42dc7