Add RPC binder as unstable API with Rust bindgen

The build targets have limited visibility to the current (and hopefully
no more) clients.

Bug: 190547489
Bug: 189947807
Test: use in the client

Change-Id: I8701066e0b44c21d2b2a77bbf872efd4e28c3662
diff --git a/libs/binder/rust/src/lib.rs b/libs/binder/rust/src/lib.rs
index 7c0584b..cb330a6 100644
--- a/libs/binder/rust/src/lib.rs
+++ b/libs/binder/rust/src/lib.rs
@@ -119,6 +119,13 @@
 pub use proxy::{AssociateClass, DeathRecipient, Proxy, SpIBinder, WpIBinder};
 pub use state::{ProcessState, ThreadState};
 
+/// Unstable, in-development API that only allowlisted clients are allowed to use.
+pub mod unstable_api {
+    pub use crate::binder::AsNative;
+    pub use crate::proxy::unstable_api::new_spibinder;
+    pub use crate::sys::AIBinder;
+}
+
 /// The public API usable outside AIDL-generated interface crates.
 pub mod public_api {
     pub use super::parcel::ParcelFileDescriptor;