LT-15814: Msg: Error during paste. Paste has been undone

* fix string marshaling bug in Icu.Split()
* add unit test for Icu.Split()
* fix Icu.Split() so it works with all types of boundaries

Change-Id: Id169895b4acb0dbbcb5ab0e40d36207506ed0dbf
(cherry picked from commit efced4a512ee63ccf0e53777b5ba197d46fdc650)