QtQuick.Dialogs FileDialog: don't go astray into qrc:/ on Android

Was due to string->URL conversion when setting FolderListModel.folder.

Task-number: QTBUG-36006
Change-Id: Ia768d8a5473b3d4c22ef9be7c8b3cf28d3956f6f
(cherry picked from qtdeclarative/cf2b04f6da6fb4d6a7da924a0796bfa8fda25398)
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>