Implement Discovery PLATFORM logic in Rust

Add RemoteAuthConnectionCache and RemoteAuthPlatform
with support to sendMessage

Design doc: go/remote-auth-manager-fishfood-design

Test: built successfully.
Bug: : 291333048
Change-Id: I17f73b4fb2e22924a484eeb3baa9b933ae980076
diff --git a/remoteauth/service/jni/src/lib.rs b/remoteauth/service/jni/src/lib.rs
index a816c94..c6f8c72 100644
--- a/remoteauth/service/jni/src/lib.rs
+++ b/remoteauth/service/jni/src/lib.rs
@@ -21,5 +21,7 @@
 mod unique_jvm;
 mod utils;
 
+/// Implementation of JNI platform functionality.
 pub mod remoteauth_jni_android_platform;
+/// Implementation of JNI protocol functionality.
 pub mod remoteauth_jni_android_protocol;