[camera_avfoundation] Fix incorrect types in image stream events

[RobertOdrowaz]

Fixes the types used in the image stream events. Most importantly, it corrects the lensAperture to lensAperture**()** that caused a crash. Some other arguments were also updated to match exactly the types expected by the parsing in type_conversions.dart.

Resolves: flutter/flutter#170240

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[stuartmorgan-g]

Maybe it's out of scope for this PR, but now that this file is Swift the event channel could be converted to a Pigeon event channel. That would make this class of bug impossible here, because we'd have actual type checking.

[RobertOdrowaz]

Maybe it's out of scope for this PR, but now that this file is Swift the event channel could be converted to a Pigeon event channel. That would make this class of bug impossible here, because we'd have actual type checking.

I agree 100%, but I wanted to fix this ASAP because it seems to affect a number of people. I will open an issue for the pigeon refactor

[stuartmorgan-g]

I just noticed that the Pigeon generation is still Obj-C, so switching to Pigeon event channels would be a much bigger change than I had realized. Definitely out of scope here!