Revert "Revert "Delete VPN methods in ConnectivityManager.""

1. Stop using ConnectivityManager for VPNs in VpnDialogs.
2. Delete updateLockdownVpn, since all callers have been migrated
   to calling VpnManager directly.
3. Delete the call to VpnManager in factoryReset, since the only
   caller (ResetNetworkConfirm) has been updated to call into
   VpnManager directly.
4. Delete getVpnManager, since it is now unused.

This reverts commit 51bf8e97.

Reason for revert: should be safe to submit now that aosp/1596096 is merged

Bug: 173331190
Test: treehugger
Change-Id: Ife3607c024006ce4fe46c981e9742170becb6331