[macOS] Consolidate FlutterViewController static types/data

[cbracken]

No semantic changes, just consolidates static functions in FlutterViewController.mm local to the translation unit into a single contiguous block at the top.

Also includes three minor (non-semantic) changes:

• Corrects static function naming to UpperCamelCase
• Adds doc comments for OnKeyboardLayoutChanged
• Adds a mark to help highlight the static data/function block at the top of the file in Xcode.

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• I updated/added relevant documentation (doc comments with ///).
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• All existing and new tests are passing.

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[flutter-dashboard]

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[dkwingsmt]

LGTM

[stuartmorgan]

test-exempt: code refactor with no semantic change

Looks like you'll have to revert the consolidation part though; the thing you moved uses a declaration in the controller's private declaration section, but the controller's private declaration section uses a struct from the anonymous namespace.