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 Id997fb6572daa3ff55591afa1676953bb204f33a

Because of a strange Gerrit interaction, earlier patchsets were recorded
as the following separate changes (to soon be abandoned):

- I59d86ae28123890ddb7aabc7dc9a73a75eecbca4
- I9cc95266fa4500d019e09ec53ec00e5dea7ffd38
- Ie6f23a2035e160f231406df1388a43725380df1d
- Id87909e96fa7fa91174c86395a964404f3490fe1
- I57c5cbde0425c42538d589fa4d1802b3e5688f7c
- Ic1f9d7727e416da1b6c4f1a45b53750cd4088dcf
- I20a72ccc4ba1bc08ed3ee39bff71d2adb376588c
- I417f6f364144b90085f5fef4bd2de1a40b5dfa79

Bug: 6188932
Change-Id: I771e57e3b8c0eb6721c2022e775cb0b528bdff93