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.
If you are just looking at the last input buffer.. maybe you can name it like that?
this.lastInputBuffer = event.inputBuffer;