Since the managedRfc2898DeriveBytes is used to implement the one-shots on some platforms, this changes the internals of it to better support spans.

I took this approach as an alternative to a pure one-shot managed PBKDF2 implementation to use as much existing, battle tested code as possible to increase the chances that it will be taken in time for .NET 7.

I investigated using the native PBKDF2 capabilities for Android and determined that unfortunately Android's APIs are not suitable for our needs for two reasons. First, it requires API Level 26 for SHA2 PBKDF2, which means we would still need the managed fallback since we support older Android versions. Second, it operates entirely onChar[] passwords, notByte[]. We can't feed arbitrary bytes in to Java character arrays.