Fix bug #16660405 Stability: ISE in Settings: Observer ...

com.android.settings.SettingsPreferenceFragment$1@3c1d9ecb was not registered

- use a monitor to control registering / unregistering

Change-Id: Id66dd698abf92643c97938e2091c3be38e6b78bd

src/com/android/settings/SettingsPreferenceFragment.java

@@ -66,6 +66,7 @@ public class SettingsPreferenceFragment extends PreferenceFragment implements Di
     private boolean mPreferenceHighlighted = false;
     private Drawable mHighlightDrawable;
 
+    private Object mRegisterLock = new Object();
     private boolean mIsDataSetObserverRegistered = false;
     private DataSetObserver mDataSetObserver = new DataSetObserver() {
         @Override
@@ -153,18 +154,22 @@ public class SettingsPreferenceFragment extends PreferenceFragment implements Di
     }
 
     public void registerObserverIfNeeded() {
+        synchronized (mRegisterLock) {
             if (!mIsDataSetObserverRegistered) {
                 getPreferenceScreen().getRootAdapter().registerDataSetObserver(mDataSetObserver);
                 mIsDataSetObserverRegistered = true;
             }
         }
+    }
 
     public void unregisterObserverIfNeeded() {
+        synchronized (mRegisterLock) {
             if (mIsDataSetObserverRegistered) {
                 getPreferenceScreen().getRootAdapter().unregisterDataSetObserver(mDataSetObserver);
                 mIsDataSetObserverRegistered = false;
             }
         }
+    }
 
     public void highlightPreferenceIfNeeded() {
         if (isAdded() && !mPreferenceHighlighted &&!TextUtils.isEmpty(mPreferenceKey)) {