default audio output should be first in the enumerateDevice returned list

[youennf]

Following microphones and cameras, we should try exposing the default audio output device using its first position.
This also probably means it should always be exposed since '' allows to be used by setSinkId.

In case of change of default audio output device, we might also want to fire a devicechange event if more than one audio output device has been exposed.

[guidou]

I agree with this. Also, this all matches Chromium's behavior.

[guest271314]

enumerateDevices() currently returns inconsistent results re selecting and listing default devices w3c/mediacapture-main#727. Would suggest to fix fix that inconsistency. Also, there really is no "default audio output device" #111 in either this or Media Capture an Streams main, at best there is a re-routing of audio input device.

[guidou]

Since enumerateDevices is defined in mediacapture-main, I filed a https://github.com/w3c/mediacapture-main/issues/756 to track it there and closing this one.