Convert entity type checkboxes to multiselect on field settings page

It will help us create a more sane schema file.
Currently in head

  'settings' => 
  array (
    'excluded_entity_type_ids' => 
    array (
      'comment' => 'comment',
      'contact_message' => 'contact_message',
      'block_content' => 'block_content',
      'menu_link_content' => 'menu_link_content',
      'shortcut' => 'shortcut',
      'node' => 0,
      'file' => 0,
      'taxonomy_term' => 0,
      'user' => 0,
    ),
    'excluded_bundle_ids' => 
    array (
    ),
    'target_type' => 'node',
  ),

As you can see some values are string and some values are FALSE.
We don't have to store keys which are set to FALSE.
After the patch

'settings' => 
  array (
    'excluded_entity_type_ids' => 
    array (
      'comment' => 'comment',
      'contact_message' => 'contact_message',
      'block_content' => 'block_content',
      'menu_link_content' => 'menu_link_content',
      'shortcut' => 'shortcut',
    ),
    'excluded_bundle_ids' => 
    array (
    ),
    'target_type' => 'node',
  ),

and this will become the correct schema

    excluded_entity_type_ids:
      type: sequence
      label: 'Entity types to exclude'
      sequence:
        - type: string