chore(liveness): update video constraints for selected camera

[esauerbo]

Description of changes

• Remove existingDeviceId and add facingMode: 'user' to tell camera to prefer
• This fixes an issue where the back camera would be chosen for FaceMovementAndLight if it was used most recently (FaceMovementAndLight only works with front camera)

Issue #, if available

Description of how you validated changes

Deployment: https://testing.d2pnskzbmchcgk.amplifyapp.com/ Compare to: https://alpha.d3v0f004886eky.amplifyapp.com/

Selected FaceMovementChallenge and chose a back camera, then went back to FaceMovement + light and verified that front camera was used

Checklist

• [ ] Have read the Pull Request Guidelines
• [ ] PR description included
• [ ] yarn test passes and tests are updated/added
• [ ] PR title and commit messages follow conventional commit syntax
• [ ] If this change should result in a version bump, changeset added (This can be done after creating the PR.) This does not apply to changes made to docs, e2e, examples, or other private packages.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[changeset-bot[bot]]

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Latest commit: 6487531ce51c4b501fcbc50cf3ce5a63a42712b0

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