Actually set the internal modal value in ListPopupWindow.setModal()

The current code ignores the parameter and sets the internal modal
value to true. This has probably gone unnoticed because all framework
usage of this method calls it with 'true', except for one case where
it sets it to 'false' temporarily, and then to 'true'.

Issue #16815832 ListPopupWindow setModal broken

Change-Id: I4ac7ab31b4ab91c9959efd4ccf2264a8e381708f