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

[hitesh-wagento]

Original Pull Request

#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 #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)

[dmytro-ch]

This PR should be closed since it does not contain all the needed changes for Magento 2.1 version and actually does not fix the mentioned issue.
Here is the original backport to 2.1-develop branch: #15781

Thanks.

[lfluvisotto]

PR rejected, priority granted to the original PR author.

#15902 (2.2-develop) (original)

#15781 (2.1-develop) (backport)