logd: disable worst uid prune for events buffer

There is some usage statistics that would be hurt by pruning by UID,
since _all_ usage statistics come from system_server. In other words
we expect it to be chatty. Until we formulate and evaluate a better
(eg: per-tag?) filtration mechanism, lets hold off pruning by UID.

Bug: 19608965
Change-Id: Iddd45a671e13bdcf3394c20919ad1f2e4ef36616