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.
Now checking samples for channel 0 and 1 if two channels are detected, if matched samples are within epsilon 15% of the buffer size it's assumed the microphone is Mono. From the samples I've seen real stereo mic inputs usually result in 0 matching samples hence this should be good enough.