Added default video and audio modes for RTC

[thvardhan]

I've added default modes to enable and disable video and audio. This can be controlled by rtc settings props. Usage :

<AgoraUIKit settings={{
            defaultVideo:false,
            defaultAudio:true
        }} connectionData={connectionData} rtcCallbacks={rtcCallbacks} />

This will enable audio while disabling video when the user first joins. they can then click on the control buttons and enable video if required