There was a feature advertised for Android phones a few years ago (IIRC) that I always felt would be used for this kind of purpose. Basically, if the phone was

There was a feature advertised for Android phones a few years ago (IIRC) that I always felt would be used for this kind of purpose. Basically, if the phone was playing video and was no longer pointed to your face, it would pause the video. I could be wrong on the specifics here, but that seems close to what the feature advertised was.