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
diff --git a/libs/binder/rust/src/binder.rs b/libs/binder/rust/src/binder.rs
index 63c8684..976f54d 100644
--- a/libs/binder/rust/src/binder.rs
+++ b/libs/binder/rust/src/binder.rs
@@ -1091,7 +1091,7 @@
         }
     } => {
         $( #[$attr] )*
-        #[derive(Debug, Default, Copy, Clone, PartialOrd, Ord, PartialEq, Eq, Hash)]
+        #[derive(Default, Copy, Clone, PartialOrd, Ord, PartialEq, Eq, Hash)]
         #[allow(missing_docs)]
         pub struct $enum(pub $backing);
         impl $enum {
@@ -1104,6 +1104,15 @@
             }
         }
 
+        impl std::fmt::Debug for $enum {
+            fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+                match self.0 {
+                    $($value => f.write_str(stringify!($name)),)*
+                    _ => f.write_fmt(format_args!("{}", self.0))
+                }
+            }
+        }
+
         impl $crate::binder_impl::Serialize for $enum {
             fn serialize(&self, parcel: &mut $crate::binder_impl::BorrowedParcel<'_>) -> std::result::Result<(), $crate::StatusCode> {
                 parcel.write(&self.0)