Fixed stereo input and comparison

* Finally figured out how to get stereo input. Turned out the master constraint echoCancellation=false was all that was needed.
* testInputVolume() now returns the calculated db or volume for the provided buffer and channel.
* Rounded the db value as we do not more accuracy than a float without decimals.
Calling it stereo was easier to understand.

My comment was to add an else to the "if(activeChannels.length === 2) { } else { ... }".
But I see now that you print the number of channels above.