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.
could you move the testFinished in this function? And replace the reportFatal with reportError (it makes it easier to read the code flow till termination if there is only one very explicit testFinished path).