Android: fix cancellation handling (react-native-image-picker#506)

- This was broken by react-native-image-picker#496.
- Currently, the response object is empty, lacking the `didCancel` flag.

android/src/main/java/com/imagepicker/ImagePickerModule.java

@@ -307,7 +307,7 @@ public void onActivityResult(Activity activity, int requestCode, int resultCode,
 
     // user cancel
     if (resultCode != Activity.RESULT_OK) {
-      responseHelper.invokeResponse(callback);
+      responseHelper.invokeCancel(callback);
       callback = null;
       return;
     }