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/Settings.java

@@ -34,7 +34,7 @@ import java.util.List;
 /**
  * Top-level settings activity to handle single pane and double pane UI layout.
  */
-public class Settings extends PreferenceActivity {
+public class Settings extends PreferenceActivity implements ButtonBarHandler {
 
     private static final String META_DATA_KEY_HEADER_ID =
             "com.android.settings.TOP_LEVEL_HEADER_ID";