Fixes uptime (As has been checked to work in the UI for Instruments and Platform)

* uptime is now working for platform and instrument in the UI as has been verified by using the console in developer tools in chrome. Note: The mapping in the UX still needs to be updated so that it shows correctly on the UI page's top left hand corner.
* uptime correctly gives the autosample time (non zero value, if we wait a few minutes) when device is streaming/monitoring
* When device is stopped from streaming or monitoring, the uptime resets to "0 days, 0 hours, 0 minutes"
* Makes the get_uptime() method generic so that it handles both instruments and platforms