Sensor event time should come from sensor event

[graychan]

the title says it all.

also examine whether this approach produces more accurate sensor event time.

[SagarPandit01]

I think I got it to work;

    // write, timestamp of the sensor event here.
    mChannel.write("," + event.timestamp);    //added by Sagar Pandit on March 24, 2017
    mChannel.write("\n");