Use the formatted time for preview clock time

Current issue: b/157440597, the AMPM field is available on the preview screen in Mandarin, even we removed it.

Screenshot of fixing:
lockscreen 12-hour: https://screenshot.googleplex.com/Ub59KfE8nzJ.png
lockscreen 24-hour: https://screenshot.googleplex.com/UiPb2Xvtxx2.png
theme preview 12-hour: https://screenshot.googleplex.com/uHMovJwu8GG.png
theme preview 24-hour: https://screenshot.googleplex.com/Kxwr6Loqy5e.png

Test: Manually
Fixes: 157440597
Change-Id: I6be7ddb5cb33ae9f5f57f4847728f3d75c5100da