[EZ] Correctly classify whether `this` type is annotated
[EZ] Correctly classify whether `this` type is annotated

Summary:
Changelog: [internal]

The original code seems to incorrectly flip whether the `this` type is annotated or inferred. While we can just flip it back, it's good to use `annotated_or_inferred_of_option` from type util, and to be consistent with similar code in [`class_sig`](https://github.com/facebook/flow/blob/c0ac59c23944a95d2ec82bd8997a2885953cc71f/src/typing/class_sig.ml#L867).

Reviewed By: jbrown215

Differential Revision: D36684076

fbshipit-source-id: 18a2542a6212e1bf73f882ae9489193b10793d77

mroch/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow