sound-player: Don't deadlock in finish_cb

The function finish_cb can be called as a result of a call to ca_context_cancel
in cancelled_cb. This will result in a deadlock because, as per documentation,
g_cancellable_disconnect cannot be called inside the cancellable handler.

It is possible to detect if the call to finish_cb is caused by ca_context_cancel
checking if error_code == CA_ERROR_CANCELED. To avoid the deadlock we should
call g_signal_handler_disconnect instead g_cancellable_disconnect if this is the
case.

!474
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