Fragmentize WifiSettings.

- Add button bar feature toward SettingsPreferenceFragment,
  which has existed in PreferenceActivity and has been used
  (probably) only by Settings app.
- super.onActivityCreated() is not called at the beggining of
  WifiSettings#onActivityCreated(), the parent method assumes
  the child should have prepared PreferenceScreen, while
  WifiSettings cannot do until the parent Activity is ready.
- Call SetHasOptionMenu() should be called AFTER the parent
  Activity is ready. It is not documented, so it would be better
  to file another bug.
- Add exception to proguard...

Change-Id: Iebd27f0cb0abdbee9b4b1cc9b00f4bf127f7815d