Switch Settings from IConnectivityManager to VpnManager.

The VPN code is moving out of ConnectivityService to a new
VpnManagerService. Update Settings to call into the VpnManager
class instead of using the AIDL interface directly. This way,
the VPN code can be moved without touching Settings.

Bug: 173331190
Test: builds, boots
Test: manually verified VPN settings page
Change-Id: Id2731a166b5d6783acb1c711a54604b69aa8c0d7