[native] Fix up issues from D13175

Summary:

1. Prettier does not run on flow-typed to avoid applying changes to libdefs that come from upstream. As such, it's important that we manually fix up whitespace issues.
2. In libdefs, we should always be specific between {| |} and { ... }, and never just use {}
3. We should prefer type to interface whenever possible to avoid complex inheritance-based typing
4. We should maintain consistent style across the file. This diff replaces the inconsistent ; that was introduced in D13175 with , that matches the rest of the file
5. We should always makes types read-only unless they need to be written to

Test Plan: Flow

Reviewers: inka, tomek

Reviewed By: inka, tomek

Differential Revision: https://phab.comm.dev/D13263