[WifiSetup] Use default theme for Wi-Fi dialogs

Just use the default theme as the dialog doesn't require any
additional attributes to be specified. (Theme.Settings is setting
alertDialogTheme to Theme.AlertDialog as well).

This will allow the correct accent colors (among other theme
attributes) to be propagated to the dialog.

Bug: 20663720
Change-Id: Ibb12c185e8104ccb8f3ddd01524127c748b47487