[FIX] ReplaceableConverter prevented @XStreamAsAttribute

The annotation @XStreamAsAttribute only works with
SingleValueConverters, but the logic of XStreamExtensionProvider wrapped
such converters, i.e. they were presented to XStream as normal Converter
implementations.

Effectively, this transformed all User fields of Activity classes from
attributes to full tags.

In addition, this corrects the unregistering mechanism for converters in
XStreamExtensionProvider.

Change-Id: I7369327a0d3445c168d07d392532cef6caace635
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1571
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>