Possible additions to fields checked by DecimalFormat::operator==

Per #5186 the checks for ICU4C DecimalFormat::operator== were made much more complete.

There are some additional variables it might be worth comparing in some circumstances (depending on other settings in the object):

• fCurrencyChoice

• fAffixesForCurrency & fPluralAffixesForCurrency

This ticket is to determine whether and when these should be compared, and to add any necessary checks.