JNI: Explicitly convert QtJniType instances to jobject for variadic arguments

When calling the (old and outdated) variadic argument overloads of
callMethod (as opposed to the variadic template overloads), then we
cannot pass complex types.

That types declared via the Q_DECLARE_JNI_CLASS macro are trivial was an
implementation detail that we shouldn't rely on as long as those types
are not part of the public API. Cast explicitly to jobject so that we
can make those types proper QJniObjects.

Updating this code to use the new variadic template overloads is for a
follow-up commit.

Change-Id: Ia9d610c84de6f989ef957c23401b1cbc64d2d1ef
Reviewed-by: Petri Virkkunen <petri.virkkunen@qt.io>
Reviewed-by: Assam Boudjelthia <assam.boudjelthia@qt.io>

src/plugins/platforms/android/qandroidplatformdialoghelpers.cpp

@@ -19,7 +19,7 @@ static jclass g_messageDialogHelperClass = nullptr;
 
 QAndroidPlatformMessageDialogHelper::QAndroidPlatformMessageDialogHelper()
     : m_javaMessageDialog(g_messageDialogHelperClass, "(Landroid/app/Activity;)V",
-                          QtAndroid::activity())
+                          static_cast<jobject>(QtAndroid::activity()))
 {
 }