https://www.da.vidbuchanan.co.uk/blog/android-c2pa.html is both funny and inevitable. Hardware-backed key storage is intended to prevent the private key material being moved to a different system, not prevent the private key material being used in ways you don't like on the system it's stored on. You are not going to stand a good chance of preventing the device owner using that key material however they want (which is why all the fearmongering about TPMs enabling meaningful DRM has always been bullshit)