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: I98f5cf1d18b8de151ea84fb16fc359b39a8bb4ab