Update futures-task to 0.3.21 am: c8d44d99a8 am: 703f74a90e am: c42af5446a

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/futures-task/+/2004793

Change-Id: I71778f17ede6d18206eb1e534b515568d187a094