Yes, some are quite sensitive. I did some experiments with an iPhone 5S, wanting to detect respiration. Was surprised that it also clearly picked up heartbeat. It works best with the phone laying on your stomach while laying on your back. It takes some filtering to get a clear signal, but iOS (and I presume Android) has the necessary signal processing APIs to clean it up.
Please don't try this while asleep and unmonitored, and certainly not with a Samsung!