Make android context getter nullable and add listeners for change

> Wouldn't we throw a nice exception if jni related code requires an android context object and it wasn't initialized? (It shouldn't crash, hang or otherwise - it should provide a nice error message explaining the problem and how to address it)

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

No files or tests are named. Start by locating the Android context getter and the JNI-related initialization path, then determine the expected nullable behavior and listener contract from surrounding APIs. Done should be defined by the issue's requested getter and change notifications, along with a clear error path when JNI code lacks an initialized context.

Written by the indexing model from the issue text.