Android: Use less direct access to the qmake build step

Instead of qobject_cast use the reacently introduced virtual function
BuildConfiguration::regenerateBuildFiles() to trigger the necessary
work in the qmake case.

Change-Id: Ie5fd36550b5bad8c5f3f8bb1e2a3a16843f8f5b7
Reviewed-by: Vikas Pachdha <vikas.pachdha@qt.io>

src/plugins/android/androidmanifesteditorwidget.h

@@ -191,6 +191,7 @@ private:
     QTimer m_timerParseCheck;
     TextEditor::TextEditorWidget *m_textEditorWidget;
     AndroidManifestEditor *m_editor;
+    QString m_androidNdkPlatform;
 };
 } // namespace Internal
 } // namespace Android