[UIMA-3969] Rename the Static Final constant for CDATA (to follow Java naming conventions) and move it to the outer class.  Move the numChildren and its getter to the inner class, because the outer class is supposed to be for reusable configuration info only (and therefore thread safe after configuration).  This is only used by a test case I think; update the test case to find it in the inner class.  Rename the inner class from "XmiCasDocSerializer" to just "CasDocSerializer" because it will be used for both Json and XMI styles of serialization.

git-svn-id: https://svn.apache.org/repos/asf/uima/uimaj/branches/2.6.0-json@1617368 13f79535-47bb-0310-9956-ffa450edef68