Add metadata files for either. am: 27f5168c66 am: 7ca9b7c31e am: 506d75e22c

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/either/+/1621404

Change-Id: Ia9f6290c96dc6403eb78bd6713868c4924684e3a