title: getLastKnownLocation method always returns null
body: <p>I have a problem to fixed position. I use <code>getLastKnownLocation(LocationManager.NETWORK_PROVIDER);</code>
  but always return null, i have set the permissions in your <code>AndroidManifest.xml</code>.</p>
  
  <pre><code>&lt;uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" /&gt;
  &lt;uses-permission android:name="android.permission.ACCESS_MOCK_LOCATION" /&gt;
  </code></pre>
  
  <p>and enabled in Settings --> Location and Security --> location through network.</p>
  
  <pre><code>TextView locationShow = (TextView) this.findViewById(R.id.location);
      double latitude = 0.0, longitude = 0.0;
      LocationManager locationManager = (LocationManager) getSystemService(Context.LOCATION_SERVICE);
      if (locationManager.isProviderEnabled(LocationManager.GPS_PROVIDER)) {
          Location location = locationManager.getLastKnownLocation(LocationManager.GPS_PROVIDER);
          if (location != null) {
              latitude = location.getLatitude();
              longitude = location.getLongitude();
          }
      }
      else {
          LocationListener locationListener = new LocationListener() {
              public void onLocationChanged(Location location) {
                  if (location != null) {
                      Log.i("SuperMap", "Location changed : Lat: " + location.getLatitude() + " Lng: " +
                          location.getLongitude());
                  }
              }
  
              public void onProviderDisabled(String provider) {
              }
  
              public void onProviderEnabled(String provider) {
              }
  
              public void onStatusChanged(String provider, int status, Bundle extras) {
              }
          };
          locationManager.requestLocationUpdates(LocationManager.NETWORK_PROVIDER, 1000, 0,
                                                 locationListener);
          Location location = locationManager.getLastKnownLocation(LocationManager.NETWORK_PROVIDER);
          if (location != null) {
              latitude = location.getLatitude();
              longitude = location.getLongitude();
          }
          locationShow.setText("经度：" + latitude + "纬度：" + longitude);
  </code></pre>
  
  <p>I find that other apps can show the location correctly, so maybe there something wrong with my code.</p>