Move Gallery Benchmarks out of `flutter/flutter` and into `flutter/gallery`.

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Is your feature request related to a problem? Please describe.
Goal: Move Gallery benchmarks out of flutter/flutter and into flutter/gallery.

Describe the solution you'd like
See flutter/flutter#66351

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

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Will be unblocked by flutter/packages#232

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Closed by #367