Update futures-channel to 0.3.21 am: f7dc9c1e42 am: 99a38b94a8 am: d97505630b

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/futures-channel/+/2005931

Change-Id: I8291a944aff1c10ab0bd2cf9f7508080a56795b7