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