Remove / add feature for NotSendSyncAcrossAllPlatforms

[Fabus1184]

If NotSendSyncAcrossAllPlatforms is just there in case of some eventual future Android compatibility, why not lock it behind a feature flag? Not being able to use streams across threads is a major drawback compared to the compatibility with the not yet supported Android AAudio API.

https://github.com/RustAudio/cpal/blob/f063271c232a4447169376e0c2be0c8a50649985/src/platform/mod.rs#L66-L72