[Android] Implement the http authentication in xwalk default client.

The implemented are to provide the http anthentication funtions
required by xwalk default client, covering the following:

1 Implement the class of HttpAuthDatabase.
2 Initialize the instance of HttpAuthDatabase.
3 Implement the method of setHttpAuthUsernamePassword().
4 Implement the method of getHttpAuthUsernamePassword().

BUG=#784, #787