ticket:10017: Add new createInstance() method to C++ ListFormatter to accomodate durations.

git-svn-id: http://source.icu-project.org/repos/icu/icu@33642 251d0590-4201-4cf1-90de-194747b24ca1
ticket:10017: Allow UTMUTFMT_FORMAT_STYLE_NUMERIC as style in the constructor.

git-svn-id: http://source.icu-project.org/repos/icu/icu@33643 251d0590-4201-4cf1-90de-194747b24ca1