[camera_avfoundation] Implementation swift migration - part 3

[RobertOdrowaz]

Migrates camera implementation as part of flutter/flutter#119109

This PR prepares for the next big one (FLTCam class migration):

1. Extracts a util method for creating a test FLTCam instance. After migration, the FLTCam constructor is marked with throws so it requires some asserts to ignore it in tests
2. Removes dependence on KVO in StreamingTests

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