Add Trusty rules.mk file am: 8453e54e2e am: 0d2d5da6b8 am: 3c95e7e9a6

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/downcast-rs/+/2770001

Change-Id: I9065256a62af4a32f603cc34a6a701e92c513dd6
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>