LOK: let doc_getDocumentType() use supportsService()

css::frame::XModel::getArgs() may or may not return a Sequence that
contains a DocumentService key, while
css::lang::XServiceInfo::supportsService() can always determine the
document type.

This fixes the problem that doc_getDocumentType() returned
LOK_DOCTYPE_OTHER for Writer documents on Android.

Change-Id: I380d59a963553fb30a3eb20fbe84dcfc6a1bbd61