Hide `JObjectPtr` and others for a cleaner namespace

[HosseinYousefi]

It might be good idea if JObjectPtr etc.. are in separate import file than JniObject and friends, so less stuff in jni namespace by default, and autocomplete for JO only shows JObject and not JObjectPtr.

Not sure if that's possible to do neatly at this point.

Originally posted by @mahesh-hegde in https://github.com/dart-lang/jnigen/pull/126#pullrequestreview-1174036122