logd: Worst Tag filter enabled for events buffer

- Add drop logistics to TagTable
- replace uid references to a key reference since it
  is an UID for most buffers, but a TAG for the
  events and security buffer
- template the find worst entry mechanics into LogFindWorst class

Bug: 30118730
Change-Id: Ibea4be2c50d6ff4b39039e371365fed2453f17a2