Improve thumbnail listener disablement

When disabling thumbnail computation thanks to the feature at NXP-28743, asynchronous computation is skipped, but the synchronous listener (firing the async event) is still fired.
2 problems:

• it is unnecessary to fire the async listener when disabling computations anyway
• the synchronous listener still adds facets while computation is supposed to be disabled --> tests at org.nuxeo.ecm.restapi.server.jaxrs.management.TestThumbnailsObject show that the facet is actually needed for later re-computation.

--> add another check in the synchronous listener, using the same constant in the event context.