Extract TetherInterfaceSM to its own class.

( cherry-pick of e3f93b02 )

Attempt to keep all existing logic in place, except:

 + Marked a constructor as public, rather than default visible.
 + Added TAG, DBG, VDBG, and decoder ringer statics.
 + Moved static constants related to USB IPs into TetherInterfaceSM.

Bug: 28833951
Test: WiFi Tethering works on angler.

Change-Id: Id961220a9045832354cfe7381e5e9c0d8f54bf90