Return "not implemented" error from BindSocketToNetwork properly.

Previously, was only checking the Android SDK version. But it also needs
to check for the presence of the connectivity manager service.

BUG=webrtc:7026

Review-Url: https://codereview.webrtc.org/2697943002
Cr-Commit-Position: refs/heads/master@{#16631}

webrtc/sdk/android/api/org/webrtc/NetworkMonitor.java

@@ -114,6 +114,12 @@ public class NetworkMonitor {
     nativeNetworkObservers.remove(nativeObserver);
   }
 
+  // Called by the native code to determine if network binding is supported
+  // on this platform.
+  private boolean networkBindingSupported() {
+    return autoDetector != null && autoDetector.supportNetworkCallback();
+  }
+
   // Called by the native code to get the Android SDK version.
   private static int androidSdkInt() {
     return Build.VERSION.SDK_INT;

webrtc/sdk/android/api/org/webrtc/NetworkMonitorAutoDetect.java

@@ -469,6 +469,10 @@ public class NetworkMonitorAutoDetect extends BroadcastReceiver {
     }
   }
 
+  public boolean supportNetworkCallback() {
+    return connectivityManagerDelegate.supportNetworkCallback();
+  }
+
   /**
    * Allows overriding the ConnectivityManagerDelegate for tests.
    */