Complete the fix for cache issue due to the currencies with no symbol

[dmytro-ch]

Description

Using getCurrencySymbol() leads to a bug for currencies where there is no currency symbol - cache ends up being non-unique. Using currency code for caching is a more foolproof way.

The issue has been partially fixed in 2.2-develop branch in #13894 (Magento\ConfigurableProduct\Block\Product\View\Type\Configurable)
and #14559 (Magento\CatalogWidget\Block\Product\ProductsList).

Manual testing scenarios

1. Test the Catalog Category View page (including configurable products)
2. Test the Catalog Product List widget

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 @orlangur, thank you for the review.
ENGCOM-1953 has been created to process this Pull Request

[magento-engcom-team]

Hi @dmytro-ch. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.2.6 release.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.