logd: worst uid record watermark part four

(cherry pick from commit 831aa297)

With part deux we caused an apparent regression by not checking for
stale recorded iterators. This checking was on-purpose bypassesed
when leading prune entries were to be deleted without touching the
statistics engine due to an in-place merge.

Part deux had us leaving iterators we were not focussed on untouched
which in turn because they were left behind, had a much higher
likelihood of being deleted without touching the statistics engine.

Perform the check every delete.

Bug: 23789348
Bug: 23490267
Change-Id: Idc6cc23d1f9e3b6cd9a083139a0de59479fbfe08