Avoid NPE by fixing ResearchLogger initialization

Previously, mMainResearchLog and mMainLogBuffer were set up
when the user moved to a new TextView, and set to null when
the user left the TextView.  This change causes
mMainResearchLog, mMainLogBuffer, mFeedbackLog, and
mFeedbackLogBuffer to be non-null forever after init() is
called.  start() no longer sets up these fields; instead
they are cleared and reset every time stop() is called.
Checks for null values are now removed.

The earlier code just didn't initialize these variables if
the user disabled logging, but since the new version
invariantly keeps these variables valid, we add a check for
whether the user has enabled logging in publishLogUnits().

Change-Id: Ifde3517f1cf924cfa33cda95fec24529b52b3c08