[Android] Integrate extension system with XWalkExtensionAndroid.

 * Enable XWalkCoreExtensionBridge as a bridge.
 * Implement unregisiter method for android xwalk extension.
 * Remove API version from XWalkExtension class.
 * Remove the code stub of sample extension.
Merge pull request #660 from gaochun/extension_management

Integrate XWalk extension system with XWalkExtensionAndroid.