[VCN10] Add new API to listen for highest score network

Test: atest ConnectivityServiceTest#testRegisterBestMatchingNetworkCallback
Bug: 175662146
Change-Id: Ifa411c7b53da789c74fff7e1a95f9c9ebf5bd05c

tests/net/java/com/android/server/ConnectivityServiceTest.java

@@ -11026,4 +11026,12 @@ public class ConnectivityServiceTest {
         verifyNoNetwork();
         mCm.unregisterNetworkCallback(cellCb);
     }
-}
+
+    @Test
+    public void testRegisterBestMatchingNetworkCallback() throws Exception {
+        final NetworkRequest request = new NetworkRequest.Builder().build();
+        assertThrows(UnsupportedOperationException.class,
+                () -> mCm.registerBestMatchingNetworkCallback(request, new NetworkCallback(),
+                        mCsHandlerThread.getThreadHandler()));
+    }
+}