[Backport] Act better on existing input focus instead of removing it

[mageprince]

Original Pull Request

#13989
The focus of the input field is maintained, proper classes are added and autocomplete fires once the component is initialized. Fixes #13988.

Description

As already mentioned in issue referenced above this change proposes better approach when the user is already focused on the search input by detecting it and triggering appropriate methods.

Fixed Issues (if relevant)

1. Mini search field looses focus after its JavaScript is initialized #13988 Mini search field looses focus after its JavaScript is initialized

Manual testing scenarios

1. Load Magento 2 shop.
2. Focus on mini search form field.
3. Check that input no longer loses focus once its JavaScript is initialized.
4. Check that proper active classes are applied when the user is focused on the search field and JavaScript gets initialized.

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 @mageprince. Thank you for your contribution.
We will aim to release these changes as part of 2.1.15.
Please check the release notes for final confirmation.