Add demonstration unittest for TetherInterfaceSM

Prove that the concept works by writing a trivial test.

Bug: 28833951
Test: `runtest frameworks-services -c
  com.android.server.connectivity.tethering.TetherInterfaceSMTest`

Change-Id: I51911cef7f0d5165221845070befd085497c21a6
diff --git a/services/tests/servicestests/Android.mk b/services/tests/servicestests/Android.mk
index 59c6970..50e0662 100644
--- a/services/tests/servicestests/Android.mk
+++ b/services/tests/servicestests/Android.mk
@@ -12,6 +12,7 @@
 LOCAL_SRC_FILES := $(call all-java-files-under, src)
 
 LOCAL_STATIC_JAVA_LIBRARIES := \
+    frameworks-base-testutils \
     services.core \
     services.devicepolicy \
     services.net \