Define Utils.formatRelativeTime() and use it

Previously, relative times were formatted using formatElapsedTime()
(appending translations of "ago" to them), sometimes resulting in
grammatically hard-to-understand or unnatural localizations. Now we
use ICU's RelativeDateTimeFormatter, which uses grammatically correct
and natural localizations from CLDR data.

Bug: 64507689
Bug: 64605781
Bug: 64556849
Bug: 64550172
Test: make -j RunSettingsRoboTests
Change-Id: Ia2d098b190ab99e7748ef6f03b919f5c6174ba7d