[camera_avfoundation] Implementation swift migration - part 7

[RobertOdrowaz]

Migrates camera implementation as part of flutter/flutter#119109

This PR migrates the 4th chunk of FLTCam class to Swift:

• setZoomLevel
• setFlashMode
• flutterErrorFromNSError

Some properties of the FLTCam have to be temporarily made public so that they are accessible in DefaultCamera.

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[hellohuanlin]

sorry i just came back from OOO. Will look into it later today.