Make android context getter nullable and add listeners for change

[HosseinYousefi]

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