Add WifiPickerActivity for PICK_WIFI_NETWORK

- We disable left pane when PICK_WIFI_NETWORK intent is used.
- Add ButtonBarHandler interface and let SettingsPreferenceFragment
  use it instead of Settings. Note that
  PreferenceActivity#hasNextButton() and
  PreferenceActivity#getNextButton() are currently protected methods,
  while Settings' relevants are public.

Bug: 3355247
Change-Id: Ie0fae8a1dbce9c7ce37375dff8395dc09b1cc74e