Fix crash in PreferenceScreen.onRestoreInstanceState()

There is PreferenceActivity with SwitchScreenPreference. When you try to rotate screen, an application crashes with exception:

```javascript
Caused by: java.lang.ClassCastException: android.view.AbsSavedState$1 cannot be cast to org.holoeverywhere.preference.PreferenceScreen$SavedState
            at org.holoeverywhere.preference.PreferenceScreen.onRestoreInstanceState(PreferenceScreen.java:124)
            at org.holoeverywhere.preference.SwitchScreenPreference.onRestoreInstanceState(SwitchScreenPreference.java:123)
            at org.holoeverywhere.preference.Preference.dispatchRestoreInstanceState(Preference.java:148)
            at org.holoeverywhere.preference.PreferenceGroup.dispatchRestoreInstanceState(PreferenceGroup.java:87)
            at org.holoeverywhere.preference.PreferenceGroup.dispatchRestoreInstanceState(PreferenceGroup.java:90)
            at org.holoeverywhere.preference.PreferenceGroup.dispatchRestoreInstanceState(PreferenceGroup.java:90)
            at org.holoeverywhere.preference.Preference.restoreHierarchyState(Preference.java:935)
            at org.holoeverywhere.preference.PreferenceActivity.onRestoreInstanceState(PreferenceActivity.java:794)
            at android.app.Activity.performRestoreInstanceState(Activity.java:916)
...
```