Added proper millisecond time stamp calculation in nrTimestamp()

[gokulmadhavan25]

Probem Current implementation of nrTimestamp() depends on external flow to update the milliseconds value which is bound to be wrong values for total time in milliseconds.

Solution Use the Roku OS method GetMilliseconds() to get the current millisecond and append to the returned timestamp value.

[CLAassistant]

All committers have signed the CLA.