Change formatting for device up time.

- formatDuration() was changed to return only the largest meaningful
unit of time. For example, for 4 hours 35 mins, it will be formatted to
5 hours. Changing to use formatElapsedTime() instead to show the more
precise duration for the device up time.

Bug: 111828610
Test: make RunSettingsLibRoboTests
Change-Id: I9f9194ac179859b35b9eda6ca49f9eee9f11fe70