Use correct port for getting window_handles on Selendroid.

Fix for a bug on Selendroid backend  where ```driver.contexts``` fails
after switching to ```CHROMIUM``` context.

Here's what this bug is about:

```python
In [1]: opera.driver.contexts
Out[1]: [u'NATIVE_APP', u'CHROMIUM']

In [2]: opera.switch_to_webview()

In [3]: opera.driver.contexts
Out[3]: 
[{u'message': u'no such session\n  (Driver info: OperaDriver=0.1.289193,platform=Linux 3.13.0-33-generic x86_64)'},
 u'CHROMIUM']
```