Migrate to cargo_embargo. am: 4b1f18c90f

Original change: https://android-review.googlesource.com/c/platform/external/flatbuffers/+/2882726

Change-Id: I8557e5d7b80d785de5021a8bf1df53feb2897f42
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>