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

NOTE: If the value does not seem to correctly update, please reload the browser page. It usually needs that to get the developer tool console refreshed with the new stuff.

Another important point: When you just start streaming or monitoring with the device (and so you are still on the facepage.. for commands), the computed attributes do not get calculated and set. So at that point, you will not see them in the developer console for the browser. But if you click on the status page, at that point, these values are computed and set. Then when you refresh the browser, and try MODEL_DATA on the developer tool, you will see the correctly updated computed attributes. You will see uptime has been correctly updated.