JNI: remove implicit conversion of declared QtJniTypes to jobject

It's dangerous, as it results in implicit conversion to void * or bool,
while hiding the object life time implications.
Such objects are QJniObject subclasses, so make sure we don't end up
in the catch-all assignment operator when assigning. Constrain that
operator to types that are convertible to jobject.

Change-Id: If31761c00a678490eeb0b2b038e89e78d8a51747
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Assam Boudjelthia <assam.boudjelthia@qt.io>
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>

src/corelib/kernel/qjniobject.h

@@ -534,7 +534,8 @@ public:
     // This function takes ownership of the jobject and releases the local ref. before returning.
     static QJniObject fromLocalRef(jobject lref);
 
-    template <typename T>
+    template <typename T,
+              std::enable_if_t<std::is_convertible_v<T, jobject>, bool> = true>
     QJniObject &operator=(T obj)
     {
         assign(static_cast<T>(obj));

src/corelib/kernel/qjnitypes.h

@@ -21,8 +21,6 @@ template<typename Type>
 struct Object : QJniObject
 {
     using Class = Type;
-    operator jobject() const noexcept { return object(); }
-
     Q_IMPLICIT Object(jobject object) : QJniObject(object) {}
     Q_IMPLICIT Object(const QJniObject &object) : QJniObject(object) {}
     Q_IMPLICIT Object(QJniObject &&object) : QJniObject(std::move(object)) {}