(stable) Fix an issue where "Export Data" could fail if a user had a nonempty custom policy preference

Summary:
The "Export Data" workflow incorrectly uses the "Policy Favorites" setting to choose a default export format. This is just a copy/paste error; the correct setting exists and is unused.

If the setting value is an array (as the "Policy Favorites" value often is), we try to use it as an array index. This generates a runtime exception after D21044.

```
[2020-06-16 06:32:12] EXCEPTION: (RuntimeException) Illegal offset type in isset or empty at [<arcanist>/src/error/PhutilErrorHandler.php:263]
  #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer, array) called at [<phabricator>/src/applications/search/controller/PhabricatorApplicationSearchController.php:460]
```

  - Use the correct setting.
  - Make sure the value we read is a string.

Test Plan:
  - Used "Export Data" with a nonempty, array-valued "Policy Favorites" setting.
    - Before: runtime exception.
    - After: clean export.
  - Used "Export Data" again, saw my selection from the first time persisted.

Differential Revision: https://secure.phabricator.com/D21361