authfs: support remote output directory

This change contains 3 major groups:
 - authfs/{aidl, fd_server}: new AIDL API and the service implementation
 - authfs/src: implement FUSE APIs for creating directory and file, by
               interact with the new service API as a client
 - authfs/tests, tests/: test coverage

A few notable changes that might help reviewing:

 - Now that both AuthFs and FdService struct is no longer immutable (in
   order to allow writable directory), their BTreeMap are now guarded by
   Arc<Mutex<_>>.

   * AuthFs::insert_new_inode and FdService::insert_new_fd are designed
     specifically to allow querying then mutating the map, which isn't
     trivial.

 - File and directory modes from the user program / VFS are currently
   ignored (just not to grow the change size).

 - Some shuffling of test paths to make it easy to clean up in tearDown.

Bug: 203251769
Test: AuthFsHostTest
Change-Id: I50f3f1ba8a3ebd969cf0f25a8feab2ec8cb1a2dc
diff --git a/authfs/src/file.rs b/authfs/src/file.rs
index 404e3a5..bbe5e6c 100644
--- a/authfs/src/file.rs
+++ b/authfs/src/file.rs
@@ -1,5 +1,7 @@
+mod remote_dir;
 mod remote_file;
 
+pub use remote_dir::RemoteDirEditor;
 pub use remote_file::{RemoteFileEditor, RemoteFileReader, RemoteMerkleTreeReader};
 
 use binder::unstable_api::{new_spibinder, AIBinder};
@@ -8,9 +10,10 @@
 
 use crate::common::CHUNK_SIZE;
 use authfs_aidl_interface::aidl::com::android::virt::fs::IVirtFdService::IVirtFdService;
-use authfs_aidl_interface::binder::Strong;
+use authfs_aidl_interface::binder::{Status, Strong};
 
 pub type VirtFdService = Strong<dyn IVirtFdService>;
+pub type VirtFdServiceStatus = Status;
 
 pub type ChunkBuffer = [u8; CHUNK_SIZE as usize];