Move service-connectivity to the tethering APEX

As part of modularization of ConnectivityService and expansion of the
Tethering module scope, move service-connectivity.jar into the tethering
APEX, and load it from there.

Bug: 171540887
Test: m, device boots and connectivity
Change-Id: Id7b6a4664ae73224b9ab219c94f56d603a62ee5a

tests/net/Android.bp

@@ -70,4 +70,7 @@ android_test {
         "android.test.base",
         "android.test.mock",
     ],
+    jni_libs: [
+        "libservice-connectivity",
+    ],
 }