Store client pid within LogEvent

LogEvent now stores the client's pid (the pid defaults to -1 if it is
not known). This pid will be used internally for permission checks.

This CL also fixes a bug where uids of -1 were being treated incorrectly
as unsigned ints.

Test: bit statsd_test:* (passes when the new socket schema flag is
flipped on).

Change-Id: I8b6b202bb719b79646282764f82466c8065031b3