ResearchLogging capture full n-gram data

- Captures complete motion data for all words in an n-gram.
- Also filters n-grams properly; if any word in the n-gram is not
  in the dictionary, it is not included.
- Simplify ResearchLog to not require explicit state
- Added LogBuffer class MainLogBuffer class to allow n-gram-level decisions
  about privacy.
- Moved LogUnit out from ResearchLogger

multi-project change with Ic70ccb6c2e11eb02d887821434b44daa3eb7a3e2

Bug: 6188932
Change-Id: I731d6cff6539e997874f723b68bdb0d9b651b937