[Network] Improve JNI experience

With the recent improvements of QJniObject, it is much easier to write JNI-related code. No more long and confusing Java function signatures are needed.

See Volker's blog post for all the improvements.

I think this can be useful not only for Qt users, but also our own code can benefit from that.

This task is to apply the improvements for Network Android backend (specifically, qtbase/src/network/kernel/qnetworkproxy_android.cpp but probably something else).