Import async-trait crate. am: d1b91c7bb6 am: 10666183a1 am: 392b2dc8b0

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/async-trait/+/1400154

Change-Id: Ib0e4d4471fc962b36a131fa288bc7a031737ac1d