Better define channels in AudioConfiguration

[mounirlamouri]

It's currently hand-wavy and inconsistent with Web Audio. We should decide whether we want to keep the definition as including the sub (ie. .1) or if we want to stay fully consistent with Web Audio. I would suggest to stay consistent with DASH.

[jpiesing]

The eventual solution needs to be clear about whether channels includes downmix of multi-channel audio to stereo. It may be worth making this explicitly visible as some content providers may wish to provide their stereo audio track rather than rely on an unknown downmix.