[firebase_ml_vision] Prepare example app to include Live Camera Preview demo and Material Barcode Scanner demo

[bparrishMines]

Description

This PR reorganizes the example app to use a list that allows access to multiple MLVision examples. Including a Live Camera Preview and a Material Design Barcode Scanner.

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I signed the CLA.
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Breaking Change

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• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[bparrishMines]

This widget was moved to picture_scanner.dart and renamed to PictureScanner.