Add a registerSystemDefaultNetworkCallback method.

This method allows internal components to track the system
default network. This differs from registerDefaultNetworkCallback
because that method sends callbacks for the default network for
the UID that called it. This may not be the system default
network, for example, when a VPN is up and applies to the UID.

Bug: 173331190
Test: new unit tests in ConnectivityServiceTest
Test: new unit tests in ConnectivityManagerTest
Test: new CTS tests in ConnectivityServiceTest
Test: new CTS tests in HostsideVpnTests in other CL in this topic
Change-Id: Id02748a2183f71b71ff2a53a580466b9dcecaa93