logd: worst uid record watermark part deux

(cherry pick from commit 49afe0d0)

Only record watermark if not known, or represents the worst UID
currently under focus. This has resulted in a halving of the average
prune time in the face of heavy spam because we get less processing
spikes.

Bug: 23327476
Bug: 23681639
Bug: 23685592
Change-Id: I19f297042b9fc2c98d902695c1c36df1bf5cd6f6