Login screen and Guest mode should use device owner preference
settings.clock.use_24hour_clock value for time format.

BUG=258199

Review URL: https://chromiumcodereview.appspot.com/21133012

git-svn-id: http://src.chromium.org/svn/trunk/src/chrome/browser/policy/proto@216232 4ff67af0-8c30-449e-8e8b-ad334ec8d88c