Convert `DeviceHandle` into a struct (#4186)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4186

We already expose the only private member `VkDevice` from Adapter's public function `device_handle()` so nothing's really private. This change also makes it consistent with the layout of `PhysicalDevice` and allows decoupling from Adapter in the next change.
ghstack-source-id: 233085716
exported-using-ghexport

bypass-github-export-checks
bypass-github-pytorch-ci-checks
bypass-github-executorch-ci-checks

Reviewed By: SS-JIA

Differential Revision: D59540129

fbshipit-source-id: ced3cd5f6e40546caabfcd024a2d3225753ebf71