Extracts backend-specific code in DlSurfaceProvider to separate translation units. In particular, this allows for less conditional header includes, and more specifically, allows code relating to the Metal backend to include headers that include ARC-managed Objective-C types. Today we cast these all to void* (and manage refcounting manually) since these headers are included in dl_surface_provider.cc, which is a pure C++ translation unit.
No test changes since this patch includes no semantic changes.
Issue: https://github.com/flutter/flutter/issues/137801
[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
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