Don't show context menu for list header in VolumeBrowserActivity
Workaround for null MenuInfo in onContextItemSelected()

MenuItem.getMenuInfo() is returning null in onContextItemSelected() due
to proxying showContextMenu() through ListView.setOnItemLongClickListener().
This is a workaround to save the position of the currently selected list
item in the long click listener and use that in onContextItemSelected().
Ignore long presses on the (..) item too