[Android] Support the tag member in W3C WebNotification API

(defined at http://www.w3.org/TR/notifications/ )

Fixed bug XWALK-1968: The “tag” member of Notification does
not work when running “WebAPI/Notification” on Android OS
in java code, 
`if (something) dosomething`
or
```
if (something) {
    dosomething
}
```