1. If I have my phone on and recording, the police can seize it (ie, grab it from me). At best, I'd be able to turn it off, but it's been shown multiple times that they can force me to provide my fingerprint to unlock it.
2. If I turn my phone off (so they need my pin, which they can't force from me), I can't record.
The phone is encrypted - but the battery isn't removable, and I assume there's more than one way to get at the data as long as the phone stays on (usb/pin header access, some form of cold boot attack, kernel 0day).
I seem to recall the iPhone used to be similar wrt using the camera on a locked device - you can disable the touch id and enable a more secure pin? (although, if it's still possible to brute force the hsm, going to a "secure" number of digits isn't really practical. But six digits with three attempts before actually hardware enforced cool down should help a bit. But even at just 1 guesses / 3 seconds on average, it'd at worst take a month (15 days on average) to get into such a device, assuming an automated/robot brute force toolkit - especially with faulty tamper resistant hw).
Even if you do prove they stopped you from recording, so what? You don't have evidence of your interaction to implicate them. As long as they recite the magical words, that they believed they were in the right, current legal doctrine will look the other way(see Heien vs North Carolina - https://blog.simplejustice.us/2014/12/16/heien-v-north-carol... - and similar).
You just start recording them, and then if they try to stop you, their attempt to stop you is now recorded, and hopefully in the cloud where they can't delete it.