Update the NetworkAgent API for council comments

Bug: 152238712
Test: FrameworksNetTests NetworkStackTests
Copy from ag/10824609
Change-Id: I9a2691f783f4449348c3f767568e05620f0b9df5
Merged-In: I9a2691f783f4449348c3f767568e05620f0b9df5

core/java/android/net/SocketKeepalive.java

@@ -109,6 +109,16 @@ public abstract class SocketKeepalive implements AutoCloseable {
     })
     public @interface ErrorCode {}
 
+    /** @hide */
+    @Retention(RetentionPolicy.SOURCE)
+    @IntDef(value = {
+            SUCCESS,
+            ERROR_INVALID_LENGTH,
+            ERROR_UNSUPPORTED,
+            ERROR_INSUFFICIENT_RESOURCES
+    })
+    public @interface KeepaliveEvent {}
+
     /**
      * The minimum interval in seconds between keepalive packet transmissions.
      *