iOS: Refactor ShellTestPlatformViewMetal (#56370)

Migrates DarwinContextMetal to a plain C struct, eliminating the need for constructor, getters, etc. since it's only used in this translation unit. The fields themselves cannot be inlined as fields on ShellTestPlatformViewMetal because the header in which that class is defined is included in plain C++ (non-Obj-C++) translation units and therefore cannot contain Obj-C types.

This simplifies the code and simultaneously fixes complicated ARC behaviour in which the const "DarwinContextMetal::context()" getter caused retainCount to be incremented on the underlying context_ pointer, but not decremented even if never unassigned. In particular the line
```objc
FML_CHECK([metal_context->context() mainContext]));
```
appeared to cause refcount to be incremented but never decremented.

Regardless of the ARC issue, this significantly simplifies the code.

This also eliminates the last remaining use of fml::scoped_nsobject in Flutter's codebase. That class will be removed in a followup PR.

No test changes since there is no change to semantics.

Issue: https://github.com/flutter/flutter/issues/137801

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style