[Rlog56] Buffer words before pushing out LogUnit

Previously, a logbuffer only held an n-gram.  Data went in and out of it, FIFO, until privacy
conditions were met (i.e. data not collected too frequently), and then an n-gram was saved.
E.g., if n=2, and only 10% of data is collected, then 18 words went through the logbuffer before
it captured the next 2 words.

However, if a user then went back and edited the n-gram, these edits were not captured.

This change changes the logbuffer size to temporarily hold data about words that are not recorded,
so that if the user backs up over them, the edits to an n-gram that we do eventually capture are
stored.  If the example above, instead of a logbuffer holding 2 words, it holds 20.  The system
waits until all the words not needed for the n-gram have been gathered (i.e. the buffer is full),
so the user has adequate time to edit, before shifting out the n-gram.  The buffer is still flushed
when the user closes the IME.  See the comment for MainLogBuffer for an explanation.

multi-project commit with I45317bc95eeb859adc1b35b24d0478f2df1a67f3

Change-Id: I4ffd95d08c6437dcf650d866ef9e24b6af512334