Objective-C files should be named to match their header, in this case, //flutter/shell/platform/darwin/ios/framework/Headers/FlutterPlatformViews.h.
When private API is required, we often create a header named FlutterFoo_Internal.h, but the implementation file is always just FlutterFoo.mm. This brings FlutterPlatformViews back into line with convention.
No test changes since this is just a file rename.
[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
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