logd: correct for number of elements in prune

(cherry pick from commit 58b8be8906f903ac3d83c41bcb0fb9c7841945f0)

Chatty logs would distort the average log size by elevating the
elements, but not the size. Add statistical collection for the
number of elements that report chatty, and subtract that from
the number of elements to improve the pruning estimate. Pick
minElements as 1% rather than 10% of the total with this more
accurate number of elements, to a minumum of 4.

Bug: 24511000
Change-Id: I3f36558138aa0b2a50e4fac6440c3a8505d95276
4 files changed