Bug 1748718 - Part 2: Use the specified type for rust xpidl constants, r=emilio

Previously all xpidl constants were specified as `i64` which means they require
casts before being passed to any xpcom methods. The lack of typing was not an
issue in c++ due to implicit casts from enums to integer types, but using the
correct type is much more valuable in Rust.

Differential Revision: https://phabricator.services.mozilla.com/D135165