Move pageVisibility state and monitoring code into the browserInfo utilities module

[rtibbles]

Currently our page visibliity state tracking code is part of our core vuex state: https://github.com/search?q=repo%3Alearningequality%2Fkolibri%20pageVisibility&type=code

This should be migrated to the browserInfo utilities module: https://github.com/learningequality/kolibri/blob/develop/packages/kolibri/utils/browserInfo.js

Instead of vuex state, we should create a VueJS ref to track the state in a reactive way, and export it from the module.

Any existing references to the pageVisibility state should be updated to reference this new export from the module.

Ideally, these would be done by returning the value from the setup function of any components using it.