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.
