1) don't be a dick to cops, and NORMALLY they won't be a dick to you. As much as you don't want to admit it, cops are just like everyone else. Outside of work, they get drunk, hang out with their buddies, beat their wives and secretly loath their kids, just like everyone else. They understand what your going through. If your gonna record them, just tell them. no problems.
2) I'm not sure about others, but I have a run in with the police, oh, once every 3-5 years maybe, and that's because I got caught speeding. I'm not sure what you all are doing that causes you to have enough contact with the police that you need an app for that, but you might want to reevaluate your life choices if you feel like you need one.
3) never talk to the police outside of giving them your name. they aren't there because they want to be your buddy, they are there because they are collecting evidence. Use this test. If you know something that you don't want the cops to know, ask for a lawyer, don't talk to the police.