Do not destroy socket when VPN interface address is still in use

Normally when an IP address is removed, all sockets associated with the
addresses are destroyed. This patchset changes this behavior such that
if the address in question is still being used by another interface that
belongs to the same underlying virtual network, the destroy operation is
skipped. This change is needed to support VPN seamless handover where the
VPN app will establish a second TUN interface (with different config)
before tearing down the existing interface. The intention is that during
this handover existing socket connections should not be disturbed. There
is a companion change in the framework side to make sure during such
handover, the VPN netId remains unchanged so routing still works.

Bug: 64692591
Test: cts-tradefed run commandAndExit cts-dev -m CtsHostsideNetworkTests -t com.android.cts.net.HostsideVpnTests
Test: system/netd/tests/runtests.sh
Change-Id: I02c6b0db5f15cd1aef3e3fa6f0c36e86b4f427fd