Add flag to disable part of event_logging in Settings.

- Fork SettingsEventLogWriter from EventLogWritter. This is identical to
the base class except it overrides some logging method with a flag
check.  We can't entirely disable event_log logging because some of the stuff
being logged will not exist in stats_log, for example paired metadata.

Bug: 129697155
Test: robotest
Change-Id: I9cd46bbb54cb85737bbf7760e315c1fe34f77498