Migrate to cargo_embargo. am: 8d0b6101a8 am: 158f81c623 am: 23e7c28f17

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/downcast-rs/+/2824674

Change-Id: I776afe904009125e158df7bfd0a12232be717b69
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>