Bug 1767346 - Use browsing context activeness rather than renderLayers to determine process priority. r=mccr8,mconley,geckoview-reviewers,agi

For desktop this should basically have no impact (maybe impacts tab
warming, but if we wanted we could set the priority hint from the tab
switcher the same way we set renderLayers), but Fenix always has
renderLayers as true, effectively, so we were never de-prioritizing the
background tab processes.

Differential Revision: https://phabricator.services.mozilla.com/D145351