logd: worst uid record watermark

(cherry pick from commit c892ea3f)

Hold on to last worst uid watermark and bypass a spike to O(n*n*x)
(n=samples, x=number of spammers) wrt chatty trimming.

Bug: 23327476
Change-Id: I9f21ce95e969b67e576417a760f75c4d86acf364