[NS05] Feed network offer callbacks

The design is very simply expressed :
An offer is needed for a request if and only if that offer
might beat the satisfier for that request.

The implementation of "might beat" is NetworkRanker#mightBeat.

Test: FrameworksNetTests FrameworksWifiTests NetworkStackTests
Bug: 167544279
Change-Id: I0fe911eef2483ecbac48c733d56283b81538690a

framework/src/android/net/ConnectivityManager.java

@@ -3345,7 +3345,7 @@ public class ConnectivityManager {
      * @param score The prospective score of the network.
      * @param caps The prospective capabilities of the network.
      * @param callback The callback to call when this offer is needed or unneeded.
-     * @hide
+     * @hide exposed via the NetworkProvider class.
      */
     @RequiresPermission(anyOf = {
             NetworkStack.PERMISSION_MAINLINE_NETWORK_STACK,
@@ -3368,7 +3368,7 @@ public class ConnectivityManager {
      *
      * @param callback The callback passed at registration time. This must be the same object
      *                 that was passed to {@link #offerNetwork}
-     * @hide
+     * @hide exposed via the NetworkProvider class.
      */
     public void unofferNetwork(@NonNull final INetworkOfferCallback callback) {
         try {