Return stats from inactive ifaces, packet counts.

When an iface goes inactive, xt_qtaguid maintains historical counters
instead of discarding.  Now reading and returning those values in
NetworkManagementService summary calls.  Tests to verify stats are
returned.

Modify NetworkStats to include rxPackets and txPackets values, and
move to Entry to eventually hide internal storage details.

Bug: 4984539
Change-Id: I1ba0bb5580c62e946d9dff2cc2a6f64aff403efc