I have been a juror on a criminal case that relied heavily on phone records. There was no fingerprint system involved, and yet it was clear beyond reasonable doubt that the phone belonged to the defendant.
The phone company can testify that the phone corresponds to a given cell number. Other people can testify that they spoke to the defendant on that number. No one piece of evidence exists in a vacuum; all the pieces of evidence combine to paint a picture.
Sure you could try to lie by making up stories about how the phone wasn't really yours, but it's hard to get all the other evidence to line up with a lie. This is a GOOD thing; the protection against self-incrimination isn't intended to help people get away with murder, it's intended to protect the accused from being compelled to help the government prosecute them.
I'm not contending the "wasn't really yours" point. Phones get stolen, misplaced, left behind, etc. with enough frequency that "it's your phone and it registered/triangulated with this position" may be strong circumstantial evidence but still isn't proof. That it was used in a manner requiring your finger (still attached to your body) does.
Don't get me wrong, I'm with you on protecting the accused from compulsion to self-incrimination. Just observing that the fingerprint sensor, coupled with the enormous data being collected on/about the device, isn't helping 5th Amendment rights.
Since it's trivial to show whether or not that is true, I would not advise making such a thing. It is highly inadvisable to lie to the police or courts.
The evidence used to prove when you bought the phone would also likely be used to demonstrate that you own the phone. I don't think the fact that your fingerprint unlocks the phone found in your pocket at the time of arrest is likely to be the jury's determining factor in deciding whether that phone belongs to you.
