Update futures-core to 0.3.21 am: cc5e534819 am: 93f97b5a52 am: 40528bf6b7

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/futures-core/+/2005971

Change-Id: I5049c3c6caf24e0858a68129a10030cb811dadb2