[Impeller] Document and slightly refactor `ResourceManagerVK` & friends. (#45474)

@chinmaygarde specifically, let me know if I violated any correctness you were looking for, or if you would prefer I _don't_ make 1 or more of the changes I made here.

@gaaclarke would love your input on C++ style specifically, or anything that stands out in general.

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## Summary

I plan to use the resource strategy in https://github.com/flutter/flutter/issues/133198, but wanted to understand the code better first, so this is my contribution to make it a bit easier to understand (hopefully! push back if not!) and contribute tests.

1. Renamed `Reset(ResourceType)` to `Swap()` for consistency with the std smart pointers.
2. Moved non-trivial work out of the constructor into `::Create` ([style guide](https://google.github.io/styleguide/cppguide.html#Doing_Work_in_Constructors)).
3. Added some `FML_DCHECK`s to private APIs to enforce correctness.
4. Made classes final and methods private where they were effectively that ([style guide](https://google.github.io/styleguide/cppguide.html#Inheritance)).
5. Added tests to make sure I understood the contracts.
6 files changed
tree: 3607f76c5ed363054f476f3378a287b2f16a47d9
  1. .github/
  2. assets/
  3. benchmarking/
  4. build/
  5. ci/
  6. common/
  7. display_list/
  8. docs/
  9. examples/
  10. flow/
  11. flutter_frontend_server/
  12. flutter_vma/
  13. fml/
  14. impeller/
  15. lib/
  16. runtime/
  17. shell/
  18. sky/
  19. testing/
  20. third_party/
  21. tools/
  22. vulkan/
  23. wasm/
  24. web_sdk/
  25. .ci.yaml
  26. .clang-format
  27. .clang-tidy
  28. .gitattributes
  29. .gitignore
  30. .pylintrc
  31. .style.yapf
  32. analysis_options.yaml
  33. AUTHORS
  34. BUILD.gn
  35. CODEOWNERS
  36. CONTRIBUTING.md
  37. DEPS
  38. Doxyfile
  39. LICENSE
  40. README.md
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