[ML17] Add a method to switch the most probable language

Also directly hold a ref to it. It's simpler for now, though
we may want to remove it completely later to have a flat,
purely weight-based system with no preference.
Also split weights between gesturing and typing.

Bug: 11230254
Change-Id: Ica15964a193ae6e7f14c85a9a3ed806130fa7efb