Return offloaded traffic when querying from TrafficStats API

TrafficStats API are being used for querying realtime network
statistics for years. However, on certain devices, some network
traffic are produced by hardware components and not be
able to seen by kernel counters.

Thus, include statistics for those missing network traffic is
necessary. Note that the included statistics might be stale
since polling newest stats from hardware might impact system
health and not suitable for TrafficStats API use cases.

Test: atest FrameworksNetTests TetheringTests
Bug: 16229221

Change-Id: I6741c41cb5145ca8748f9b083b9c15e7e2735681