Remove unused method calls of MockVpn

The following are removed:
1. mUnderlyingNetworkInfo and the related get and set.
2. Calls to setEnableTeardown() since this has no
   interaction with ConnectivityService.
3. Calls to updateState().
4. Usage of mInterface.
5. Usage of mConfig.
6. Usage of mNetworkAgent.

Bug: 230548427
Test: atest FrameworksNetTests
Change-Id: Ie7cec6fed25f841de995059deba1c1df67377e70