Generate apex_available from c2a instead of a patch am: 38038e844b am: 131ed12dcb am: 3b09b8bb08

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/downcast-rs/+/1688687

Change-Id: I85ff3fcc3e4e5da8ffd73d4f62d8b47f5116a044