logd: start cleaning up LogReaderThread

1) We can use real member functions with std::thread and
   std::function, so use those instead of the 'me' pointer.
2) Don't expose member variables directly.
3) Rename and document member variables, since all of their references
   are being touched anyway.

Test: logging unit tests
Change-Id: I9a357a3ea8691433d58687c95356b984b83e9c36