[WifiSetup] Progress when wifi scanning

Use a custom empty view to show the status while wifi is scanning
or off. Cannot use android.R.id.empty because in phone portrait
layout the illustration is the listview header, which will be hidden
if the list is empty and we use ListView.setEmptyView.

Bug: 17183006
Change-Id: Ie8598303f4e61f87e078008fb7a03918ba92c9ef