[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. The return
type of shouldInterceptedRequest() is also updated accordingly.

BUG=https://crosswalk-project.org/jira/browse/XWALK-653