Hide Keepalive offload API in IpSec

Since the new keepalive API is exported in ConnectivityManager,
hide Keepalive API in IpSec, also make system-current.txt sync
for this part.

Bug: 114151147
Test: m -j
Change-Id: I076030bdbab1cd7d69f6a034577d529970b050dc

core/java/android/net/IpSecTransform.java

@@ -257,7 +257,6 @@ public final class IpSecTransform implements AutoCloseable {
      *
      * @hide
      */
-    @SystemApi
     public static class NattKeepaliveCallback {
         /** The specified {@code Network} is not connected. */
         public static final int ERROR_INVALID_NETWORK = 1;
@@ -288,7 +287,6 @@ public final class IpSecTransform implements AutoCloseable {
      *
      * @hide
      */
-    @SystemApi
     @RequiresPermission(anyOf = {
             android.Manifest.permission.MANAGE_IPSEC_TUNNELS,
             android.Manifest.permission.PACKET_KEEPALIVE_OFFLOAD
@@ -331,7 +329,6 @@ public final class IpSecTransform implements AutoCloseable {
      *
      * @hide
      */
-    @SystemApi
     @RequiresPermission(anyOf = {
             android.Manifest.permission.MANAGE_IPSEC_TUNNELS,
             android.Manifest.permission.PACKET_KEEPALIVE_OFFLOAD