[KA05] Export keepalive offload api for IpSec Nat-T file descriptor

Adds system api of createSocketKeepalive to take file descriptor,
so privileged apps could use it without the need of IpSecService.

Bug: 114151147
Test: atest FrameworksNetTests
Change-Id: If926c21704b6ed73a0adfcadad732b97b42bacae

tests/net/java/com/android/server/ConnectivityServiceTest.java

@@ -3778,6 +3778,7 @@ public class ConnectivityServiceTest {
         // TODO: 1. Move this outside of ConnectivityServiceTest.
         //       2. Add helper function to test against newSingleThreadExecutor as well as inline
         //          executor.
+        //       3. Make test to verify that Nat-T keepalive socket is created by IpSecService.
         final int srcPort = 12345;
         final InetAddress myIPv4 = InetAddress.getByName("192.0.2.129");
         final InetAddress notMyIPv4 = InetAddress.getByName("192.0.2.35");