logd: report last prune memory overhead

An estimate based on chatty impact for all known pids, uids and tags
and per log id if applicable, calculate the maximum last pruned
watermark iterator map usage and add to the Total Overhead.

Test: Confirm that the Total Overhead change is negligable.
Bug: 31942525
Change-Id: Icd2e9bc0747c3376ca0e9c90aa110c103529d98f