Avoid recursion if logging lock is corrupt.

If the logging lock is corrupt then using LOG fails and recursive death ensues.
Make Mutex a friend of LogMessage so that it can do raw logging in this special
situation.
Opportunistically also make fields of LogMessage and LogMessageData that can be
const, const.

Change-Id: I9e0d07c3224096bcf03d6410cd64bb8b5c831fac