[Android] Implement default xwalk client in runtime core.

1  Add the Ssl alert title to the file of android_xwalk_strings.grd.
2  Create a new class XWalkDefaultClient extended from class XWalkClient.
3  Create the method of onReceivedError() to handle the process of
   receiving error.
4  Create the method of onReceivedSslError() to handle the process of
   receiving ssl error.
Merge pull request #695 from ShDavidliu/master

[Android] Implement default xwalk client in runtime core.