[KA11] Verify fd ownership and allocate resource for NattKeepalive

Since socket keepalive APIs for UDP encapsulation sockets are
public to generic app. In order to ensure the given fd is valid,
this change verifies the resource id inside the UDP
encapsulation socket by using methods provided by IpSecService.

Bug: 125517194
Fix: 123968920
Test: 1. atest FrameworksNetTests --generate-new-metrics 10
      2. atestcom.android.server.ConnectivityServiceTest \
         #testNattSocketKeepalives --generate-new-metrics 100
Change-Id: I408aacc19b364683854d15a095c34e72389a6e5b