rust: Fix Debug implementation for declare_binder_enum!

Debug for Rust enums prints the variant name, while the
derived implementation from declare_binder_enum! previously
used the structure format since our AIDL enums are newtypes
over integers. The output looked something like Foo(0).
This replaces the derived Debug with a custom implementation
that prints the variant name just like Debug for Rust enums.

Test: atest rustBinderTest
Change-Id: I03597afa50918dc7a45a5c9fe288aad6dd159fb5