Use real date for theme preview

Screenshot:
  - Launcher date: https://screenshot.googleplex.com/kVY7RxFra4s.png
  - Preview date: https://screenshot.googleplex.com/DaMKPJKentj.png

Also update status bar clock to accept 12/24HR format
  - 12HR: https://screenshot.googleplex.com/jWreQpk0UjU.png
  - 24HR: https://screenshot.googleplex.com/Rb5Kwh7hdTa.png

Test: Manually
Bug: 146475648
Change-Id: I8b59609b31e328ebde15a62ac0198c963be05b05