DngCreator: Improve BlackLevel, fix thumbnail metadata

 - Use new dynamic black level result key when available
 - Write rational black level instead of integral value
 - Fix incorrect thumbnail BitsPerSample metadata
 - Fix incorrect date/time format string
 - Fix unknown orientation

Test: dng_validate for captured DNGs is clean
Bug: 27452979
Bug: 25862995
Change-Id: I87b73f7a4f4ecc3cf6498c7db16df68dd0b0b43a