admin checkout agreement controllers refactor

[AnshuMishra17]

Description

Refactor the code by removing the direct use of ObjectManager and includes the dependencies using Constructor Dependency Injection

Manual testing scenarios

1. Created, updated and deleted checkout agreements from the admin-end.

[magento-engcom-team]

Hi @AnshuMishra17. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

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[magento-engcom-team]

Hi @miguelbalparda, thank you for the review.
ENGCOM-2210 has been created to process this Pull Request

[sidolov]

Please, inject dependencies in classes when they are really needed.
Make new properties private and try to avoid usage leading underscore in var and methods name.

[AnshuMishra17]

This has been fixed.

[magento-engcom-team]

Hi @sidolov, thank you for the review.
ENGCOM-2210 has been created to process this Pull Request

[magento-engcom-team]

Hi @AnshuMishra17. Thank you for your contribution.
We will aim to release these changes as part of 2.2.7.
Please check the release notes for final confirmation.

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