Import async-trait crate. am: d1b91c7bb6 am: 10666183a1 am: 392b2dc8b0 am: 33736938d2

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

Change-Id: Ife7f2ee8267d3acaee06a0a4b16637f4698e9479