[camera] Implement video capture for CameraX android camera re-write

[gmackall]

This PR implements video capture for the CameraX re-write of the android camera plugin.

https://github.com/flutter/flutter/issues/111138

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• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• [x] I signed the CLA.
• [x] The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• [x] I listed at least one issue that this PR fixes in the description above.
• [ ] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• [x] I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
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• [x] I added new tests to check the change I am making, or this PR is test-exempt.
• [ ] All existing and new tests are passing.

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[reidbaker]

before continuing my pass I need the cla to be figured out.

[gmackall]

Heads up that I have created a new version of this pr with commits up to this point squashed to fix the CLA issue @reidbaker pointed out. Leaving this open temporarily while the outstanding comments are unresolved.

https://github.com/flutter/packages/pull/3899

[gmackall]

Merging currently blocked due to a test failure that I believe is unrelated and should be resolved by https://github.com/flutter/packages/pull/3947

[gmackall]

To make reviewing easier (because this has been updated a lot), since your last review @reidbaker the summary of changes is:

1. Added logic in startVideoRecording to explicitly do nothing if recording is not null (there is an active recording).
2. Added a test for this case where we call startVideoRecording twice, and verify the methods we expect to be called only once are indeed called only once (covering the above case).
3. Made a change to the stopVideoRecording error case where the videoOutputPath is null so that we clean up the recording objects (stop the recording, set recording and pendingRecording to null) because we can't recover from this case without starting a new recording as videoOutputPath is set in the startVideoRecording. This way change (1) doesn't get us stuck forever in the null videoOutputPath case.
4. Added a test to cover the above.

[camsim99]

Merging manually to unblock https://github.com/flutter/packages/pull/3878