fix: optimize scroll position observer after video fullscreen exit

[ihor-romaniuk]

This merge request contains an optimization for toggling video xblock full-screen mode and saving the previous window top offset position on exit from the full-screen state.

With this refactor, we call scroll after exit full-screen mode only and not depends on xblock resize because sometimes the resize event comes before exiting full screen mode, and we don't need to call scroll every time on resize.

Related PR to master branch: #1371
Related PR to redwood branch: #1405

[openedx-webhooks]

Thanks for the pull request, @ihor-romaniuk! Please note that it may take us up to several weeks or months to complete a review and merge your PR.

Feel free to add as much of the following information to the ticket as you can:

• supporting documentation
• Open edX discussion forum threads
• timeline information ("this must be merged by XX date", and why that is)
• partner information ("this is a course on edx.org")
• any other information that can help Product understand the context for the PR

All technical communication about the code itself will be done via the GitHub pull request interface. As a reminder, our process documentation is here.

Please let us know once your PR is ready for our review and all tests are green.

[itsjeyd]

OSPR management note: Build seems fine except for codecov token issues. Marking as ready for review.

[openedx-webhooks]

@ihor-romaniuk 🎉 Your pull request was merged! Please take a moment to answer a two question survey so we can improve your experience in the future.