Better logging infrastructure in OffloadHardwareInterface

Test: as follows
    - built
    - flashed
    - booted
    - "runtest frameworks-net" passes
    - watch "adb shell dumpsys connectivity tethering | tail -60" shows logs
Bug: 29337859
Bug: 32163131

Change-Id: Id0bd14e5b364c497403c2d88f7b1c31d712e6f87