Do not rely on flutter bindings being initialized for context

> What would it take to make android context objects available to JNI code without having to rely on flutter bindings being initialized? (Tangential: Also thinking about `Isolate.run()` / `compute()` running things in helper isolates which may not have bindings being setup)

_Originally posted by @mkustermann in https://github.com/dart-lang/http/issues/1241#issuecomment-2202258028_