LandingPage field types typehint the content service implementation

Most of the block classes of the `ezsystems/landing-page-fieldtype-bundle` package are typehinting the code ContentService implementation (`eZ\Publish\Core\Repository\ContentService`) instead of the interface (`eZ\Publish\API\Repository\ContentService`).

That makes impossible to decorate the `ezpublish.api.service.content` service with another class when using this.

At least the following classes are impacted:

• EzSystems\LandingPageFieldTypeBundle\FieldType\LandingPage\Model\Block\BannerBlock
• EzSystems\LandingPageFieldTypeBundle\FieldType\LandingPage\Model\Block\GalleryBlock
• EzSystems\LandingPageFieldTypeBundle\FieldType\LandingPage\Model\Block\PlacesBlock
• EzSystems\LandingPageFieldTypeBundle\FieldType\LandingPage\Model\Block\VideoBlock