Android: Add private ctors in QJNIObjectPrivate that takes va_list.

NewObjectV() was failing because QJNIObject was calling the variadic
constructors with a va_list, which in turn created a new va_list.

Change-Id: I1cf4c8133f237596964177271a20ca651174e695
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>