Add a private method to retrieve the header list from PreferenceActivity

This is required by Settings app to be able to update the header list
if the accounts list changed.

Bug: 6685701
Change-Id: If27f83ec86fe1f3519a6e25de4e4fe4864133dc8

core/java/android/preference/PreferenceActivity.java

@@ -650,6 +650,14 @@ public abstract class PreferenceActivity extends ListActivity implements
                 && mPreferenceManager == null;
     }
 
+    /**
+     * Returns the Header list
+     * @hide
+     */
+    public List<Header> getHeaders() {
+        return mHeaders;
+    }
+
     /**
      * Returns true if this activity is showing multiple panes -- the headers
      * and a preference fragment.