Bug 1091679 - Update test_receive_call_with_locked_screen to wait for th...

...e plivo call to end
I don't think we should kill the conference call here as we're still in the test. I would simply [hang up](https://github.com/mozilla-b2g/gaia/blob/master/tests/python/gaia-ui-tests/gaiatest/apps/phone/regions/call_screen.py#L106) like a normal user would do.