Share srcs in rust_defaults. am: 5107b8713f am: fd0a4e0849 am: c54c4b3ac1

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/futures-io/+/1366176

Change-Id: Ic6e9ae41cf75cec31a359283301b1463e92110af