﻿[Android] Enable the io thread client in xwalk content.

  The io thread client is implemented while not enabled in xwalk content,
this fix is to enable the io thread client in xwalk content and adding the
related handler into the xwalk content native layer and java layer.

BUG=https://crosswalk-project.org/jira/browse/XWALK-653
Merge pull request #1309 from ShDavidliu/io_thread_client

[Android] Enable the io thread client in xwalk content.