[Core: Date/Time] 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 Core: Date/Time Android backend (specifically, qtbase/src/corelib/time/qtimezoneprivate_android.cpp but probably something else).