Collect and persist tethering stats.

Use new "gettetherstats" netd command to retrieve statistics for
active tethering connections.  Keep tethering poll events separate
from UID poll, even though they end up same historical structures.

Bug: 5244846
Change-Id: Ia0c5165f6712c12b51586f86c331a2aad4ad6afb
7 files changed