GStreamerSharp: refactoring to reuse a method in PlayerEngine.AudioSinkBin

The nested class PlayerEngine.AudioSinkBin already contains a method
to get the request pad of audiotee, so it's better to reuse it from
all the places where it does the same.
GStreamerSharp: prefer an InvalidOperationException than a NRE

If retrieving the tee pad doesn't work, it's better to throw an
InvalidOperationException right away, instead of causing a NRE
at a later stage.