Use `Self` to refer to type names

[woxtu]

Have you read the Contributing Guidelines?

Issue N/A

Describe your changes

This PR changes to refer to type names using Self to resolve the following SwiftLint warning:

Prefer Self in Static References Violation: Use `Self` to refer to the surrounding type name (prefer_self_in_static_references)

Ref: https://realm.github.io/SwiftLint/prefer_self_in_static_references.html