Fix omitting manual type for arrow functions

Summary:
Changelog:
[flow-api-translator] - Fixed omitting manual type for arrow functions

Introduced in the previous arrow functions diff (D68160295) the manual type is omitted and void is generated as the return type instead.

Given subsequent JS code:

```js
export class Foo {
    foo: (val: string) => number = (val: string) => {
       return 1;
    };
}
```

the flowToFlowDef should generate:

```js
export declare class Foo {
    foo: (val: string) => number;
}
```

This is the current output (before the diff):

```js
export declare class Foo {
    foo: (val: string) => void;
}
```

Reviewed By: pieterv

Differential Revision: D69923039

fbshipit-source-id: 5a71102889fcc235ce4cb36b8c50435964c31109

tools/hermes-parser/js/flow-api-translator/__tests__/flowToFlowDef-test.js

@@ -412,6 +412,26 @@ describe('flowToFlowDef', () => {
            static foo: () => void;
          }`,
       );
+      await expectTranslate(
+        `export class A {
+           foo: (val: string) => number = (val: string) => { return 1 };
+           static foo: (val: string) => number = (val: string) => { return 1 };
+         }`,
+        `declare export class A {
+           foo: (val: string) => number;
+           static foo: (val: string) => number;
+         }`,
+      );
+      await expectTranslate(
+        `export class A {
+           foo = (val: string): number => { return 1 };
+           static foo = (val: string): number => { return 1 };
+         }`,
+        `declare export class A {
+           foo: (val: string) => number;
+           static foo: (val: string) => number;
+         }`,
+      );
     });
     it('method', async () => {
       await expectTranslate(

tools/hermes-parser/js/flow-api-translator/src/flowToFlowDef.js

@@ -1170,7 +1170,10 @@ function convertClassMember(
         );
       }
 
-      if (member.value?.type === 'ArrowFunctionExpression') {
+      if (
+        member.value?.type === 'ArrowFunctionExpression' &&
+        member.typeAnnotation == null
+      ) {
         const [resultTypeAnnotation, deps] = convertAFunction(
           member.value,
           context,