[Android]  Implement the user agent feature in runtime.

  This patch provides the capability to customize the user agent for
Android port, it implements 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