Refactoring RsTypeKind::format_as_... methods.

After recent fixes for b/214244223, the supported constructor bindings
will only use `&mut` references for `__this` parameter.  Since `*mut`
will never appear in supported bindings, we can simplify
`format_as_this_param_for_constructor_thunk` function to only cover
references.  And as a consequence, we can rename this function into
`format_mut_ref_as_uninitialized`.

This CL also shortens the name of the (somewhat related)
`format_as_self_param_for_instance_method` function into
`format_as_self_param`.

PiperOrigin-RevId: 426223281