[Android] Embedding API: getRemoteDebuggingUrl

Add api to get web socket url for remote debugging.

BUG=https://crosswalk-project.org/jira/browse/XWALK-2763
```
if (wsUrl == null || wsUrl.isEmpty()) return null;

try {
    return new URL(wsUrl);
} catch (MalformedURLException e) {
    return null;
}
```