[ET-VK] Replace `uint16_t` with `int` for swiftshader (#6788)

As title. This is a follow up from https://github.com/pytorch/executorch/pull/6675. Replace `uint16_t` with `int` as well as 16 bit vectorized types.

Differential Revision: [D65824204](https://our.internmc.facebook.com/intern/diff/D65824204/)

ghstack-source-id: 253112193
Pull Request resolved: https://github.com/pytorch/executorch/pull/6781

Co-authored-by: Stephen Jia <ssjia@meta.com>