Fixed data-saved stats getting stuck at 0.

• YouTube now streams most videos over SABR, which the extension wasn't detecting - so video quality was never actually reduced and no savings were counted. Detection now reads the player's own stream info instead of relying on network sniffing.
• Audio Only Mode now takes effect immediately when switched on - no more reloading the tab for it to start working.
• Works on youtube.com and music.youtube.com.