Generate apex_available from c2a instead of a patch am: 38038e844b am: 131ed12dcb am: 3b09b8bb08 am: 30238f23cf

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

Change-Id: Ie4f11e9cdd029f4796d8e096af0bbc2a8f2937a1