[Android] Clean term AwContent in IO thread Client and remove AwURLRequestJobFactory in cookie_manager.h.

  In io thread client related header files, the AwContents comments should be refined
and in the cookie manager header file, unused code of AwURLRequestJobFactory should be removed.
This fix is to resolve these issues.

BUG=https://crosswalk-project.org/jira/browse/XWALK-768