writerfilter: sort namespaces in OOXMLStreamImpl::getFastParser()

The motivation is that <namespace-alias> elements in model.xml are
redundant, as the same info is available from oox as well. But without
sorting, it's impossible to generate the same output, as the (not
interesting) order isn't the same there.

Change-Id: I634c62e43d1b54100bfa623c6f43dddd34279fb1