Do not disconnect existing VPN if seamless handover fails

If error happens during establishing of VPN, the cleanup logic tries to
disconnect the partially-created VPN network. At the moment this is not
working during VPN seamless handover since in this case no new network is
created and the existing VPN will be disconnected. Fix by not disconnecting
during the VPN handover case.

Test: cts-tradefed run commandAndExit cts-dev -m CtsHostsideNetworkTests -t com.android.cts.net.HostsideVpnTests
Bug: 110238496
Change-Id: I8cb2ab50596e61b0ac09e89eb1048816f00180d6