Fix issue #15895, audio streams don't signalling finished after the first one

Created by: robfram

Fix issue #15895 (closed), audio streams don't signalling finished after the first one if the audio player is set to play again, due to the order of calls in _notification. First it emits the signal, and later it disable the internal processing regardless what the callback did.

Changed to emit the signal at the end to ensure the changes done at callback remains.