Switch SystemUI from IConnectivityManager to VpnManager.

The VPN code is moving out of ConnectivityService to a new
VpnManagerService. Update SystemUI to call into the new service.

Bug: 173331190
Test: manually verified key displayed in status bar when VPN comes up
Change-Id: Id2731a166b5d6783acb1c711a54604b69aa8c0d7