nlecomposition: Deactivate current stack in PAUSED_READY state
To avoid a race when tearing down the composition (PAUSED_TO_READY), we should make sure to tear down the current stack and let the GstBin class handle the remaining thing to do during the change state. We should still ignore any error happening when tearing down the bin state just in case. https://bugzilla.gnome.org/show_bug.cgi?id=775051