hello_xr does not gracefully clean up Vulkan device/instance

`VulkanGraphicsPlugin` should destroy the `VkDevice` and `VkInstance` it owns on destruction rather than leaking them. This allows the Vulkan validation layers to detect resource leaks, which is useful for finding bugs both in hello_xr itself and in runtimes that use hello_xr with the validation layers as a smoke test for correctness.

Locate hello_xr's VulkanGraphicsPlugin and trace where it owns or creates the VkDevice and VkInstance. Verify that destruction releases both, then run hello_xr with Vulkan validation layers; done means the layers report no device or instance leaks.

Written by the indexing model from the issue text.