Remove visibility detector workaround and bump min Flutter version to 3.7.0

[jjliu15]

Description

• Removes the workaround that was added in Visibility detector workaround for android impression issue #610, which was resolved in Flutter 3.7.0.
• Sets minimum Flutter version to 3.7.0. This is aligned with the other official Flutter plugins, which support the latest two Flutter stable releases

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I updated/added relevant documentation (doc comments with ///).
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Breaking Change

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