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 quince branch: https://github.com/openedx/frontend-app-learning/pull/1370