MimeConfigurationHelper::GetDefaultFilterFromServiceName() checks that
there is both an import and an export filter, and for OOoXML the export
has been removed; tweak the check to require only import for OOoXML.

Currently it still works invoke the OOoXML filter, but it is clearly
better to store the embedded documents only in ODF, so tweak
OCommonEmbeddedObject::store* methods to do that.

(regression from aeeaccf5)

Change-Id: Ib71f23fd110cbd4b570517f2dde2c53e3aa6301d