[Android] Add the @interface JavascriptInterface.

This annotation is used for XWalkView.addJavascriptInterface. This
is for security check so all Java methods called by JavaScript code
should be marked with this annotation.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1596
Merge pull request #1876 from yongsheng/fix1596

[Android] Add the @interface JavascriptInterface.