added new flag to disable audio capture

Needed the ability to record video without audio, so I made a quick change to support it by adding a new flag, audioCaptureEnabled (following coding style appropriately using videoRenderingEnabled as a model).

Setting audioCaptureEnabled to NO (default is YES) will disable audio capture for video.

I also made changes to properly handle the error handling for audio when disabled and to handle the switch mode case correctly. Tested changes with the test app on two devices.