logd: Add worst pid of system filter

- Add a new statistic that reports per pid and log_id for AID_SYSTEM
- Add a new pruning filter ~1000/! boolean
- Use this new statistic to prune on worst pid within AID_SYSTEM

Bug: 26029733
Bug: 21615139
Bug: 22855208
Change-Id: Iab5dd28f807dcf03d276372853883f3b6afa8294