declarative-camera: FFmpeg backend chooses lower framerate for recordings

This occurs when running the declarative-camera on an iPad Air running iOS 17.5.1 and boot the example, alternating between the FFmpeg and Darwin backends. The issues occurs when doing a recording on each of the two backends.

When running the Darwin backend, the saved recording will have a framerate of 28 FPS. Meanwhile, the FFmpeg backend will save a recording with a framerate of 18 FPS.

This has not been tested on other devices.