ANDROID: rust_binder: add BINDER_TYPE_PTR support

Implement support for the scatter-gather feature of binder, which lets
you embed pointers in binder transactions and have them be translated
so that the recipient gets a pointer that also works for them.

This works by adding a second kind of object to the offset array, namely
the BINDER_TYPE_PTR object. This object has a pointer and length
embedded. The kernel will copy the data behind the pointer, and update
the address of the pointer so that the recipient will be able to follow
the pointer and see the same data.

These objects are supported recursively. Other than the pointer in the
main transaction buffer, each buffer may be pointed at by a pointer
in one of the other buffers. This can be used to build arbitrary trees
of buffers.

Link: https://lore.kernel.org/rust-for-linux/20231101-rust-binder-v1-12-08ba9197f637@google.com/


Change-Id: I8bae5afbe9b989009fdc48cd28a76a0f2fe065ff
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Bug: 278052745