Removed old apm constraints + add new master echoCancellation constraint to turn off Chrome audio processing

echoCancellation = false is the master constraint which turns off all audio processing which is necessary for this demo to work on Windows if audio ducking is enabled (which it is by default). 
Turning off Chrome audio processing also turns off audio ducking (part of it).

Details [here](https://code.google.com/p/chromium/issues/detail?id=420625).