DO NOT MERGE: Move the connectivity tests to frameworks/base/tests/net.

This will give us a good place to put all the networking tests.

Fix: 31479480
Test: adb  shell am instrument -w -e notClass com.android.server.connectivity.tethering.TetherInterfaceStateMachineTest 'com.android.frameworks.tests.net/android.support.test.runner.AndroidJUnitRunner'  # PASS

(cherry picked from commit ecc986d4)

Change-Id: I993eeaa5dec001c39389023f355f506129b356e7