Fix crash in tst_qmltc_examples on Android

tst_qmltc_examples failed because m_accessibilityContext
could become a dangling pointer if the object in points to gets
deleted. This resulted in a crash.
Amends e0c61193ea.

Task-number: QTBUG-101865
Task-number: QTBUG-95764
Pick-to: 6.3 6.2 5.15
Change-Id: Ie85118429b1afa6e4a41f899ca065f493e166570
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Jarkko Koivikko <jarkko.koivikko@code-q.fi>
Reviewed-by: Assam Boudjelthia <assam.boudjelthia@qt.io>

src/plugins/platforms/android/androidjniaccessibility.cpp

@@ -81,7 +81,7 @@ namespace QtAndroidAccessibility
     // Because of that almost every method here is split into two parts.
     // The _helper part is executed in the context of m_accessibilityContext
     // on the main thread. The other part is executed in Java thread.
-    static QObject *m_accessibilityContext = nullptr;
+    static QPointer<QObject> m_accessibilityContext = nullptr;
 
     // This method is called from the Qt main thread, and normally a
     // QGuiApplication instance will be used as a parent.