Support seamless handover between VPN fds.

This used to work before SOCK_DESTROY allowed us to close
connections when VPNs connected and disconnected. Instead of
doing this like the old code did by registering a new
NetworkAgent, support (limited) seamless handover on the
same NetworkAgent and netId.

Bug: 64692591
Test: cts-tradefed run commandAndExit cts-dev -m CtsHostsideNetworkTests -t com.android.cts.net.HostsideVpnTests#testSeamlessHandover
Change-Id: Idad0ec5946e7eb9e1f4a13c92ea7138de6a46f16