Move `utils/` and `vk_utils` out of `api/` (#4179)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4179

Flatten directories to have matching filetree and namespace hierarchies.

It's `vkcompute::vkapi`, not `vkcompute::api::vkapi`.

Updates made easy by handy commands:
```
find . -type f -exec sed -i 's/runtime\/api\/utils/runtime\/utils/g' {} +
find . -type f -exec sed -i 's/runtime\/api\/vk_api/runtime\/vk_api/g' {} +
```
ghstack-source-id: 233085711
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Reviewed By: SS-JIA

Differential Revision: D59527915

fbshipit-source-id: 8599cd2ee0b8f2092b5c31986845e96e36a76540