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

src/com/android/settings/SettingsPreferenceFragment.java

@@ -158,11 +158,11 @@ public class SettingsPreferenceFragment extends PreferenceFragment
     }
 
     protected boolean hasNextButton() {
-        return ((Settings)getActivity()).hasNextButton();
+        return ((ButtonBarHandler)getActivity()).hasNextButton();
     }
 
     protected Button getNextButton() {
-        return ((Settings)getActivity()).getNextButton();
+        return ((ButtonBarHandler)getActivity()).getNextButton();
     }
 
     public void finish() {