Support "dumpsys connectivity tethering"

Also print current upstream interface in tethering dump output.

Test: as follows
    - build
    - flashed
    - booted
    - runtest frameworks-net passes
    - "dumpsys connectivity tethering" shows just tethering info
Bug: 32163131
Bug: 36504926
Bug: 36988090
Bug: 38152109
Bug: 38186915
Bug: 38218697
Change-Id: I7f185fd71946a032e7f815207016294892df135d