Reimplement BDateTimeFormat using ICU support

* Avoid hardcoding the format to "date, time"
* Allows using DateTimePatternGenerator to create custom formats from a
set of fields.
Add tests for BDateTimeFormat with custom formats.
Use BDateTimeFormat for deskbar clock

* don't try to cook a custom format by appending strings, and instead
let BDateTimeFormat decide on the appropriate format to use.