Notify NetworkPolicyManagerService on tethering changes

No longer do this in ConnectivityService#tether/untether.
Instead, have Tethering do it when an interface is actually
tethered.

Bug: 31405407
Test: Toggling hotspot now disables data saver

Change-Id: I9910a2e488c30c92d45f817c8f5df0fac5510de6