Add OffloadHardwareInterface.getForwardedStats() wrapper

Test: as follows
    - built
    - flashed
    - booted
    - "runtest frameworks-net" passes
Bug: 29337859
Bug: 32163131

Merged-In: I41f837e16eff9a4cd763d65155bd9b5d5d980acd
Merged-In: Id9766983a3bbd885ef0f0d3ba9e33de903939f2d
Merged-In: Id831bc99cadbd59d280b026866707dbbe3b0e542
(cherry picked from commit 9a5b02a8)

Change-Id: I0752b6e2cf1e0b9a5075f7911408528538683c84