Fix review approval check ordering

[ricardoamador]

It was found that a review author can request changes be made then later approve the pull request. However Autosubmit was not honoring the chronological order of the reviews and was incorrectly blocking reviews that succeeded the original changes requested review. We now are traversing the reviews in reverse chronological order.

Also added a check so the pr author cannot review his own pull request.

List which issues are fixed by this PR. You must list at least one issue.
Fixes flutter/flutter#125280

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[keyonghan]

LGTM with nits.