enhancement: start vad when config.vad.enable is true

[rammohan-y]

currently vad detection happens only when gather verb's minBargeinWordCount is 0, we have vad at config verb, we should start vad detection when config.vad.enable is true

[vdharashive]

The purpose of this is to start streaming audio to asr when vad is detected specially with Microsoft, which send garbage Yes or No if it get empty audio stream