base: fix ClassCastException in MenuPopupWindow

The exception happens at MenuDropDownListView.onKeyDown() in
MenuPopupWindow, the code wants to cast getAdapter() into
MenuAdapter without checking the instance of getAdapter(),
MenuDropDownListView extends ListView, ListView will wrap
its adapter into HeaderViewListAdapter when header or foot
view exists, so getAdapter() may be MenuAdapter instance if
not wrapped but will be HeaderViewListAdapter instance otherwise.

So need to unwrap the adapter to get MenuAdapter for wrapped case.

CRs-Fixed: 1057424

Change-Id: If697fc972b9f2247dc9eb4522c156830568c50b1