rust_binder: refactor context management to use KVVec

rust_binder: refactor context management to use KVVec

Replace the linked list management in context.rs with KVVec.
This simplifies the ownership model by using standard
Arc-based tracking and moves away from manual unsafe list removals.

The refactor improves memory safety by leveraging Rust's contiguous
collection types while maintaining proper error propagation for
allocation failures during process registration.

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Link: https://github.com/rust-for-linux/linux/issues/1215
Signed-off-by: Jason Hall <jason.kei.hall@gmail.com>
Link: https://patch.msgid.link/20260120134119.98048-1-jason.kei.hall@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>