Use proper variables for tooltip styles on tablet devices

[vovayatsyuk]

Description

There are variables available for tooltip customization. This commit fixes an issue
on tablet devices when trying to use them.

Manual testing scenarios

1. Open Magento/blank/Magento_Checkout/web/css/source/module/checkout/_tooltip.less file and change the values of the tooltip variables as follows (Usually we do this via theme, but it's for testing purpose only):

   @checkout-tooltip-content__background-color: #fff;
   @checkout-tooltip-content__border-color: #f2f5f7;
   @checkout-tooltip-content__active__border-color: @checkout-tooltip-content__border-color;

2. Navigate to checkout and chec the new styles of field tooltips

3. Set the width to 768px and check them again:

   Without patch	With patch

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)

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[Karlasa]

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[Karlasa]

👍

[magento-engcom-team]

Hi @Karlasa, thank you for the review.
ENGCOM-4722 has been created to process this Pull Request

[soleksii]

✔️ QA Passed

Before:

After:

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