Handle camera button based on camera status

[mattblair]

Checklist

• [x] All new and existing tests pass
• [x] I've read the Contribution Guidelines
• [x] I've read Change Limitations

Motivation and Context

When tapped, the camera button in the new action area presented the system camera UI (via UIImagePickerController) without checking camera permissions first. If permissions were not determined or denied, this presented the camera UI with a black background, without explaining why the camera wasn't working.

In addition, the camera button did not have any handling for the case were the brightness sensor was already in use, preventing the use of the camera.

Description

• Added the CameraAvailability enum and the cameraAvailability property to CaptureSessionInterruptionObserver, to provide a more nuanced and actionable result than the existing isCameraUseAllowed Bool.
• Updated the ExperimentCoordinatorViewController and TrialDetailViewController to check the new enum property, and handle denied camera permission and brightness sensor limitations before presenting the camera UI.
• Added a Settings button on the permissions denied alert which will navigate to Settings -> Privacy -> Camera when tapped.
• Allowed use of the camera while multitasking, which would have been prevented in some scenarios by using the isCameraUseAllowed Bool.
• Created strings to support the new handling of permission denied.
• Added a note about potentially unexpected values for isCaptureSessionInterrupted if it is expected to be a proxy for whether the app is running in a multitasking context or not.

Changes were tested on iOS devices running running iOS 12.4.1 and iPadOS 13, turning permissions on and off, testing with and without the brightness sensor in use, and testing in multitasking modes.

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[mattblair]

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