ticket:10375: Add \file and \brief to udisplaycontext.h headers so it works in apiref/icu4c/files.html

git-svn-id: http://source.icu-project.org/repos/icu/icu@34275 251d0590-4201-4cf1-90de-194747b24ca1
ticket:10273: support plurals with decimals in MessageFormat and PluralFormat (ported from Java r34087 & r34276)

git-svn-id: http://source.icu-project.org/repos/icu/icu@34277 251d0590-4201-4cf1-90de-194747b24ca1
ticket:10355: std::string is available on Android

git-svn-id: http://source.icu-project.org/repos/icu/icu@34278 251d0590-4201-4cf1-90de-194747b24ca1
ticket:10371: fix assertion in u_strFromUTF8()/u_strFromUTF8WithSub() code

git-svn-id: http://source.icu-project.org/repos/icu/icu@34279 251d0590-4201-4cf1-90de-194747b24ca1