Add multiRetrieve method call

Author: ydag

packages/image_picker/image_picker/android/src/main/java/io/flutter/plugins/imagepicker/ImagePickerPlugin.java

@@ -94,6 +94,7 @@ public void onActivityStopped(Activity activity) {
   static final String METHOD_CALL_MULTI_IMAGE = "pickMultiImage";
   static final String METHOD_CALL_VIDEO = "pickVideo";
   private static final String METHOD_CALL_RETRIEVE = "retrieve";
+  private static final String METHOD_CALL_MULTI_RETRIEVE = "multiRetrieve";
   private static final int CAMERA_DEVICE_FRONT = 1;
   private static final int CAMERA_DEVICE_REAR = 0;
   private static final String CHANNEL = "plugins.flutter.io/image_picker";
@@ -320,6 +321,7 @@ public void onMethodCall(MethodCall call, MethodChannel.Result rawResult) {
         }
         break;
       case METHOD_CALL_RETRIEVE:
+      case METHOD_CALL_MULTI_RETRIEVE:
         delegate.retrieveLostImage(result);
         break;
       default: