[Backport 2.1] Fix minor issues in ui export converter classes

[dmytro-ch]

Original PR: #15692

Fix minor issues in \Magento\Ui\Model\Export\ConvertToXml and \Magento\Ui\Model\Export\ConvertToCsv classes.

Description

1. Fix the issue with the dynamically defined `$filter`` property.
2. Add missing throws to PHPDocs for methods.
3. Set the correct property type in PHPDocs for the $directory. (In the constructor, we init $directory property as a result of $filesystem->getDirectoryWrite() method, which creates an instance of Magento\Framework\Filesystem\Directory\Write using the corresponding factory. So, basically, it returns WriteInterface but not DirectoryList.)

Fixed Issues (if relevant)

N/A

Manual testing scenarios

N/A

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @dmytro-ch. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.1.15 release.