DngCreator: Fix calculation of date/time stamps

Converting the sensor timestamp to date/time requires checking
what the timebase of those timestamps is; getting it wrong will
create drift that increases with device uptime.

Test: android.hardware.camera2.cts.
      DngCreatorTest#testSingleImageThumbnail
Bug: 30125412
Change-Id: Ia5db86012bc9e1c06463b8dc4434d3e063f62cf5