Support setting file mode

File mode of writable AuthFsEntry can now be changed. The mode is
maintain privately in authfs, but also pass through to fd_server.

Note that this change only aims to support getting/setting the file
mode. The mode is not currently used for ACL check.

Bug: 205169366
Test: atest AuthFsHostTest
Test: atest ComposHostTestCases
Test: composd_cmd forced-odrefresh
      # exit 80 without ART hack, with permissive SELinux
Change-Id: I2405baedae9ba2be5e84eb84d3228f7be080f8c6
diff --git a/authfs/src/file.rs b/authfs/src/file.rs
index 6353209..9bbf3ef 100644
--- a/authfs/src/file.rs
+++ b/authfs/src/file.rs
@@ -1,6 +1,8 @@
+mod attr;
 mod dir;
 mod remote_file;
 
+pub use attr::Attr;
 pub use dir::{InMemoryDir, RemoteDirEditor};
 pub use remote_file::{RemoteFileEditor, RemoteFileReader, RemoteMerkleTreeReader};