Change onPlaybackStopped back to checking for 1CH playing to the old way

Found another bug in the new code. The Mark as Watched commit (7044c6a142) changed the way it was determining whether or not the currently playing video was a 1CH video when playback stopped. The new way wasn't working as it was returning True at the very least when playback was from other addons, but might have been more frequently then that even. This commit changes it back to the old way. I'm not 100% clear why it as changed to begin with.