Set playbin2's bus to flushing when shutting down until we start playback again
Set playbin2's bus to flushing when shutting down until we start playback again

This makes sure that we never get any pending messages after we
stopped playback. Without this change, error messages after a
missing plugin message could still appear although the codec
installer was started and playbin2 was stopped already.

mediaserver2-plugins/totem-mediaserver2,mediaserver2-plugins/totem-mediaserver2