Fix ConversionService contribution override

The code of org.nuxeo.ecm.core.convert.extension.GlobalConfigDescriptor is incorrect in several respects, which prevent having an override from a second contribution:

• the update method uses != instead of equals for string comparison, and should use the base String and not go through a File object.
• the GC interval and disk cache size cannot be overridden back to 10.
• the disk cache size comparison uses the incorrect default constant.