Do not rely on flutter bindings being initialized for context

[HosseinYousefi]

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