Update futures-util to 0.3.21 am: 737dc97288 am: b9180251b3 am: e2f2e216a9

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/futures-util/+/2004175

Change-Id: Iacbebadeebe2be67366f7b2932d3b2619e3eb541