Update futures-io to 0.3.21 am: d461c4b789 am: 292079557c am: fb894cf86f am: 06571956f3

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/futures-io/+/2004792

Change-Id: Ieed22c6960cd37e9f357a41101894fa5ceb745cd