Add metadata files for either. am: 27f5168c66 am: 7ca9b7c31e am: 506d75e22c am: 85f89ad7f8

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

Change-Id: Ie4f28d17d67f1d6a624e9ee10a7855ee9d2c3d97