Now we only support a single client connection

and it cannot be initiated from the local interface address for security
reasons.
Also, there's at most one playback session active at any time, and when
it dies we shutdown the client connection altogether and signal an error
to the listener.

related-to-bug: 7139784
Change-Id: Ia8d02bc994ce9986936947ddda1f2a3dddbf5714