ticket:10574: part 1, just API and setContext/getContext implementation

git-svn-id: http://source.icu-project.org/repos/icu/icu/trunk@34886 251d0590-4201-4cf1-90de-194747b24ca1
ticket:10619: Improve usability of DateFormat's leniency control & promote BooleanAttribute control to draft for v53

git-svn-id: http://source.icu-project.org/repos/icu/icu/trunk@34888 251d0590-4201-4cf1-90de-194747b24ca1
ticket:8464: Implement Andy's comments.

git-svn-id: http://source.icu-project.org/repos/icu/icu/trunk@34891 251d0590-4201-4cf1-90de-194747b24ca1