StateManager consolidation: typing, naming, management

Remove entity_config_id?

According to Jean, we need a fallback for the demo instance.

Less risky to do it after we have the test coverage.

I will take the opportunity to remove this duplicate in tests/src/Kernel/DisplayBuilderTest.php:

#[CoversClass('\Drupal\display_builder\Entity\DisplayBuilder')]
#[CoversClass('\Drupal\display_builder\Entity\DisplayBuilder')]

Work in progress