Consolidate cleanup logic in TetherInterfaceSM.TetheredState

( cherry-pick of f54c5a93 )

This pushes all TetheredState cleanup logic into a single place.
All new unittests fail without the changes to TetherInterfaceSM.

Bug: 28915272
Test: Compiles, unittests pass, WiFi tethering continues to work.

Change-Id: Ia7bf210e00e9a54f2797baebc2e5333ce314c947