Massively refactor the structure of `onionmasq-mobile`

• This changes the structure of the onionmasq-mobile crate quite significantly by introducing an OnionmasqMobile singleton struct that contains most of the business logic.
  • This avoids the need to have multiple hacky OnceCell global variables, for example; the stuff in those has been moved into the singleton.
• We use this reorganisation to add a bunch of helper methods that clean up the code and provide opportunities for cleaner error handling.
  • Most Rust->Java JNI calls now use OnionmasqMobile::call_jni_class_method, a new function that has nice "automatic" return type casting with helpful error messages if things go wrong.
  • (This means you can treat its return value as a native Rust type, instead of having to do the result.i() thing.)
• The C native functions exposed for Java to call are now in ffi.rs, and call methods on the OnionmasqMobile singleton.
  • This means the main Rust business logic can look more like idiomatic Rust, using Rust error handling conventions for example.
  • The FFI code can then wrap this in the necessary conversion functions to get it to work with Java.
• Panic handling was moved out to a new panic_handling module, and slightly improved.
  • We now log the panic inside the panic hook, instead of waiting for it to propagate out to catch_unwind, meaning we don't miss panics that happen outside such a block.
  • (Also, failing to get a backtrace is now recoverable.)
• The AndroidScaffolding used by onion-tunnel was moved out to a new scaffolding module, and changed to use the singleton.
• We also get rid of the interface_name parameter to runProxy, since it wasn't being used any more.

I apologise for the large diff! Let me know if you have any questions, or need me to split this up into smaller patches for review.