[SYCL] Make `sycl::device` use raw `device_impl *` for its `impl`

[aelovikov-intel]

device_impls are owned by the parent platforms and are only destroyed when SYCL RT is unloaded. As such, there is no reason to pay the price of a std::shared_ptr, a raw non-owning pointers is enough.

Use a pointer and not a reference because sycl::device is assignable.