Use ICU's CLDR data for "AM" and "PM" strings.

This brings DateUtils and Time in sync with bionic, icu, WebKit,
DateFormatSymbols, Formatter, and SimpleDateFormat. And specifically
means that DateUtils now knows how to say "AM" and "PM" in Japanese.

Bug: 6719054
Change-Id: Icdd75c0865505fde4115d71025eeb7515b73da4c