Fix bug #15001610 Java crash in settings is observed while adding Google account on wiped device

- update Settings main layout to be able to show the Buttons
- make WifiPickerActivity a SettingsActivity and remove dead code
- fix reference to Buttons for using the Settings IDs instead
of the Framework ones

Change-Id: I23bfd1b8290fc007e6e80aa9a6297e55e7f6004e