[Android] Refactor the xwalk settings.

  The native layer of xwalk settings interfaces of updating everything
should grab lock, the constructor should directly initialize the base
class not to involve the member of base class. The unused codes in java
 layer of xwalk settings should be removed and the ui thread should be
handled correctly. This fix is to refactor the xwalk settings to resovle
these issues.

BUG=https://crosswalk-project.org/jira/browse/XWALK-767