Revert^2 "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.

This rolls forward the change. The original topic was reverted because
of a bad merged-in clause; this is fixed and re-verified in this topic.

Bug: 171540887
Test: m, device boots and connectivity

Change-Id: I293b09c0dc04c6ccafa30cd0f1a63efe32283604