no tuned circuit required
If all you are going to do is convert RF energy to DC power, you don't need a tuner; indeed, using one would be counter-productive. All you need is an antenna of some sort, a rectifier, and possibly a capacitor to smooth out the current flow.
The biggest drawback to this idea is that it would only trickle-charge the phone when it was turned off. In my case, I leave the phone turned on nearly all of the time, including when charging, so that I will receive any incoming calls instead of having them roll over to voice mail.