Fix NativeModuleEnumMembers type (#46222)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/46222

This value was typed as always being a string, even though it  was containing both strings and numbers in the fixtures. This was because on this line https://fburl.com/code/9j7gh4av the input type is $FlowFixMe (from the source AST), which wasn't catching that it couldn't flow into just `string`.

Changelog: [Internal]

Reviewed By: makovkastar

Differential Revision: D61830075

fbshipit-source-id: 0d5a0239d7c0209049184ca858a7ceb1ada02f79