Fix bug #16957601 Stability: ISE in Settings: Observer com.android.settings.

SettingsPreferenceFragment$1@273c8fdb was not registered

- add onUnbindPreferences() call to match onBindPreferences()
- this new method is @hide so it does not impact the APIs

Change-Id: Iee0ab8a4ecc2046f89fb96cc52af150e835f658c

core/java/android/preference/PreferenceFragment.java

@@ -258,6 +258,7 @@ public abstract class PreferenceFragment extends Fragment implements
      */
     public void setPreferenceScreen(PreferenceScreen preferenceScreen) {
         if (mPreferenceManager.setPreferences(preferenceScreen) && preferenceScreen != null) {
+            onUnbindPreferences();
             mHavePrefs = true;
             if (mInitDone) {
                 postBindPreferences();
@@ -349,6 +350,10 @@ public abstract class PreferenceFragment extends Fragment implements
     protected void onBindPreferences() {
     }
 
+    /** @hide */
+    protected void onUnbindPreferences() {
+    }
+
     /** @hide */
     public ListView getListView() {
         ensureList();