Add NetworkMonitor constants to SystemApi

NetworkMonitor lives in the NetworkStack which must only use system
APIs. This includes constants used by NetworkMonitor and captive portal
login.

Bug: 112869080
Test: m
(Cherry-pick of aosp I4879568d3fbf9435767c8d4d0ab5198d421a2f4f)

Change-Id: I8b5f0bb61c3ef0d2f33af7dd67b72ac6c5a1d1b5
diff --git a/core/java/android/net/CaptivePortal.java b/core/java/android/net/CaptivePortal.java
index ee05f28..4047068 100644
--- a/core/java/android/net/CaptivePortal.java
+++ b/core/java/android/net/CaptivePortal.java
@@ -15,6 +15,8 @@
  */
 package android.net;
 
+import android.annotation.SystemApi;
+import android.annotation.TestApi;
 import android.os.IBinder;
 import android.os.Parcel;
 import android.os.Parcelable;
@@ -28,10 +30,16 @@
  */
 public class CaptivePortal implements Parcelable {
     /** @hide */
+    @SystemApi
+    @TestApi
     public static final int APP_RETURN_DISMISSED    = 0;
     /** @hide */
+    @SystemApi
+    @TestApi
     public static final int APP_RETURN_UNWANTED     = 1;
     /** @hide */
+    @SystemApi
+    @TestApi
     public static final int APP_RETURN_WANTED_AS_IS = 2;
 
     private final IBinder mBinder;