I don't know whether this works or not, but I read a suggestion on Google Products Forum for dealing with the AT&T jingle. Maybe the jingle won't play through the speakers if there are headphones plugged in. Of course, you don't want to walk around with headphones plugged in all the time. So the suggestion was to take old headphones and cut off the jack---then plug the jack into the phone.
Maybe someone could plug a headphone into the N6 before rebooting or turning back on the N6 and report back whether the jingle plays through the speaker. If not, it might be a work around without rooting. Ridiculous, though, that carriers force people to figure out workarounds because they force this on their customers.
NOTE: Thinking about this more, I'm concerned that doing this would mean you wouldn't hear phone calls coming in---so it wouldn't be a 24/7 sort of solution. But maybe just something to ensure no interruptions during a meeting or conference---use this when you would otherwise use the mute function on the phone (in the past, before Lollipop eliminated this useful setting).