Rename `at_vec::from_owned` and `at_vec::from_slice`

`str::from_slice` and `vec::from_slice` are changed to `str::to_owned` and `vec::to_owned`. (#6389)

Replace `at_vec::from_owned` and `at_vec::from_slice` with `at_vec::to_managed_consume` and `at_vec::to_managed`.