Issue 6366 - Elemhide thread is accessing released engine

libadblockplus-android/src/org/adblockplus/libadblockplus/android/AdblockEngineProvider.java

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 33 matching lines @@
    * Unregister AdblockEngine client
    * @return `true` if the last instance is destroyed
    */
   boolean release();
 
   /**
    * Get registered clients count
    * @return registered clients count
    */
   int getCounter();
+
+  /**
+   * Get lock object to prevent AdblockEngine reference from being changed
+   * @return
+   */
+  Object getEngineLock();
 }