[Android] Implement the user agent feature in runtime.

  The user agent is not supported in runtime. this is to implement
the user agent in runtime by implementing the user agent related methods
in the native layer and in the java layer including the methods of getting
the user agent, setting the user agent and others. In addition, the related
test cases are added.

BUG=https://crosswalk-project.org/jira/browse/XWALK-595