Stop exposing generated AIDL from Tethering

We don't want new modules exposing stable aidl directly. APIs should
be defined as java @SystemApi. It also seems like nothing actually
depend on these interfaces, except one simple exception.

Bug: 147200698
Test: m
Change-Id: Ia4222fa35a9a2f3c75cebb12f75c536f27e2fe16