If you really want to produce propaganda, it's possible and plausible to build optics that will project arbitrary pixels directly onto the sensor of a phone camera. It's expensive and tricky, but it's a one-off expense and small compared to known intelligence projects. The phone can be secure, unmodified, but still be "convinced" of a false reality.
Furthermore, who controls the keys? The device manufacturers definitely will have access to them (since the signing keys need to be put in the device), and it's not that you need to convince the world that it came from my phone (where you might need to extract a secret from it), you need to convince that the key came from a phone. Once the major governments get a bunch of fake keys that would have/could have been put in a real phone of a major manufacturer, you either admit the inability to discriminate propaganda or you have to automatically distrust all media produced from all phones of that manufacturer, which won't happen, the public will look at such media and believe it or not depending on whether it matches their opinion, not depending on its signature.
Furthermore, who controls the keys? The device manufacturers definitely will have access to them (since the signing keys need to be put in the device), and it's not that you need to convince the world that it came from my phone (where you might need to extract a secret from it), you need to convince that the key came from a phone. Once the major governments get a bunch of fake keys that would have/could have been put in a real phone of a major manufacturer, you either admit the inability to discriminate propaganda or you have to automatically distrust all media produced from all phones of that manufacturer, which won't happen, the public will look at such media and believe it or not depending on whether it matches their opinion, not depending on its signature.