pause video streams when document is in background, improve mobile browser handling (#159)
* visibility changed handlers on localtrack
* add visibility changed handlers on localtrack
* handle tracks when app goes to background (mobile)
* add helper function for silence detection
* emit audioSilenceDetected event on local audio tracks, log warning
* unsubscribe event handler when stopping mediatrack
* remove todo
* pause remote video when app in background
* only pause when adaptiveStream is set
* remove debug logging
* include tablets in isMobile check