Generate apex_available from c2a instead of a patch am: a8fea1d912 am: 9b2d6414ab am: 84aa6e7a64 am: de595d83f7

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/futures-channel/+/1688765

Change-Id: I15652fd3682e5bcbc4f640b78d4f880e06aef0de