ticket:10017: Implemented 2 proposals approved on 29-May. Replace TimePeriod::forAmounts in favor of ctor. TimePeriod::size() -> TimePeriod::length()

git-svn-id: http://source.icu-project.org/repos/icu/icu/branches/tkeep@33770 251d0590-4201-4cf1-90de-194747b24ca1
ticket:10017: Mark length() method as draft ICU 52.

git-svn-id: http://source.icu-project.org/repos/icu/icu/branches/tkeep@33772 251d0590-4201-4cf1-90de-194747b24ca1