ao_pipewire: avoid deadlock when starting thread loop

loop_start starts a thread which immediately acquires the thread loop
lock. Locking the loop before loop_start can therefore deadlock the
newly created thread during initialization.

Locking the thread loop before loop_start was always incorrect, but
Pipewire commit fb49e0795c28* made the issue visible by switching
pw_thread_loop_lock to the underlying SPA loop lock.

To fix this, we just avoid starting the loop until after we're connected
to the server.

* https://github.com/PipeWire/pipewire/commit/fb49e0795c28c892116b03369785ba86c27c968d