ANDROID: rust: sync: add `Arc::into_unique_or_drop`

A dependency of the Rust linked list.

This patch is an unmodified version of a patch included in the list of
dependencies of the Rust Binder RFC [1]. It is marked ANDROID: because
it has not yet been sent to the mailing list.

Link: https://lore.kernel.org/rust-for-linux/20231101-rust-binder-v1-0-08ba9197f637@google.com/

 [1]
Bug: 324206405
Change-Id: Idec3b17d99f11768319c63039cf42955beb11cba
Signed-off-by: Alice Ryhl <aliceryhl@google.com>