Watch network subtype, tethering teardown, empty.

Watch for changes to telephony network subtype, and update iface
mapping to persist stats under correct type.  Update network stats
before removing tethering NAT rules.

Skip recording that would create empty historical buckets.  Query UID
stats before iface stats to always skew positive when counters are
actively rolling forward.

Bug: 5360042, 5359860, 5335674, 5334448
Change-Id: I8aa37b568e8ffb70647218aa1aff5195d3e44d5a