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

[hitesh-wagento]

Original Pull Request

magento#15902

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 magento#13894 (Magento\ConfigurableProduct\Block\Product\View\Type\Configurable)
and magento#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)