whitelist "snet_event_log" when turning logs off

- Before silencing the logs, ensure security logs are whitelisted
- Only touch persist.log.tag properties if they need to be, add
  stacking to the property.
- Clear persist.logd.size property when at default setting

Bug: 26178938
Change-Id: I8fdb040062dc9cbb23d7bc3c117d96ff8550c132