Pass existing Instance object to islands methods instead of loading it again

Problem/Motivation

While working on #3578469: Make instance entities aware of display buildables, i have noticed once again (without being blocked by) an issue we have since the beginning of the project.

• We already have Instance objects in ApiController thanks to Drupal automatic instantiation from routes
• However, during event management, the entity object is lost and only the entity ID is kept
• We need Instance objects for ON_SAVE (soon renamed ON_PUBLISH) events in DisplayBuilderSubscriber so we do an entity storage loads
• We need Instance objects inside each implementation of IslandEventSubscriberInterface so we do many entity storage loads

This is complicated and, I guess, costly performance wise.

Proposed resolution

• Keep the instance object all along.
• Use it in DisplayBuilderSubscriber and Islands