Add TrafficStats methods to public APIs

Add methods to public APIs for mainline support.

Bug: 139268426
Bug: 135998869
Test: atest FrameworksNetTests
      ./frameworks/opt/net/wifi/tests/wifitests/runtests.sh
      atest android.net.cts
      atest android.net.wifi.cts

Change-Id: I21a9c0dbdc79b1f1041fc9e23c6a4e1e97ecde92