fix(ios): improved accuracy of ViewPager's ScrollStateChanged API

[wwwcg]

Pre-PR Checklist

• I added/updated relevant documentation.
• I followed the Convention Commit guideline with maximum 72 characters to submit commit message.
• I squashed the repeated code commits.
• I signed the [CLA].
• I added/updated test cases to check the change I am making.
• All existing and new tests are passing.

[hippy-service]

Hi, @wwwcg. Thanks for your PR! 👏

🏷️ You can leave a comment in this PR with #help tag when you need help (e.g. some status checks run failed due to internal issue), admin team members will help asap.