Re-hide onPreCheck and unhide NET_CAPABILITY_VALIDATED.

The API review comments in http://b/21343774 point out that the
suggested use case for onPreCheck (captive portal login apps) is
not a good use case as it requires that the app always be
running.

Also, unhide NET_CAPABILITY_VALIDATED, which is useful to apps
that want to detect captive portals and network connectivity
failures.

Bug: 21343774
Change-Id: Iad7c839bcc136b0fa9581dccc5fd97a28efed4ab

core/java/android/net/ConnectivityManager.java

@@ -2162,6 +2162,8 @@ public class ConnectivityManager {
          * can automatically log in to a captive portal without user intervention.
          *
          * @param network The {@link Network} of the network that is being evaluated.
+         *
+         * @hide
          */
         public void onPreCheck(Network network) {}
 

core/java/android/net/NetworkCapabilities.java

@@ -173,7 +173,6 @@ public final class NetworkCapabilities implements Parcelable {
      * Indicates that connectivity on this network was successfully validated. For example, for a
      * network with NET_CAPABILITY_INTERNET, it means that Internet connectivity was successfully
      * detected.
-     * @hide
      */
     public static final int NET_CAPABILITY_VALIDATED      = 16;