Migrate off timeFormat12 and timeFormat24.

libcore now offers a wider variety of 12-/24-hour time formats,
so be more specific about which one we want here.

(cherry-pick of 85f60d3a03b5b5d9a0e8b8a138eb85a6b53a1eca.)

Bug: 10361358
Change-Id: I846ab7a6f84cd49e876ad21e9366aff1600e0530