[camera_android_camerax] PROOF OF CONCEPT: Use platform views for camera preview

[camsim99]

Note

Proof of concept to fix flutter/flutter#181687. Full design proposal: go/flutter-new-camerax-preview

Unrelated changes:

• There's a bug in the example app that was introduced by 34eec78 I believe; TLDR we need to reinitialize the camera when it changes to make sure the preview is up to date

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[flutter-dashboard]

This pull request is not mergeable in its current state, likely because of a merge conflict. Pre-submit CI jobs were not triggered. Pushing a new commit to this branch that resolves the issue will result in pre-submit jobs being scheduled.