Implement `Send` for `Stream` about cpal

[bushrat011899]

To me, this explanation clearly rules out an AAudio backend being Sync, but I don't see anything here preventing the use of AAudio from separate threads at separate times. So at least superficially, it seems like an AAudio backend could indeed be Send.

As an alternative, could features be used to conditionally implement Send for platforms where it is known to be safe to do so? Then this feature will be missing from Android until someone tries it (and presumably) has the prerequisite knowledge to be able to contribute.

That seems to me the easiest path forward, and there is precedent in other Rust crates to do something similar. For example, the latest Bevy release conditionally implements Send for some of its futures when not on WASM.

[marcomaida94]

Same problem here, having Send would be great