If you absolutely have to hand over your phone, turn it off completely, like hold the power button and then tap the off icon. That will dump any keys out of RAM, which is why it always requires the full password to unlock when you turn it back on. Both in terms of how your phone works and the leaks we've seen, the cracking tools the police have are overall significantly less likely to be successful when used on a phone that's been turned off and not unlocked since.

Also, IIRC iphones have a feature where they will dump at least some of the system keys from RAM if you push the lock button five times. I'd still trust fully off more but that's easier to do covertly.