The FBI is able to unlock or access data on most of the phones and computers it encounters...
https://motherboard.vice.com/read/feds-can-unlock-most-devices-they-need-to-get-into-fbi-admits
If I had a cell phone, i would be using one of these programs my friends :-)
https://veracrypt.codeplex.com/wikipage?title=android%20%26%20ios%20support
That iPhone app you linked is $0.99, doesn't have any reviews, and promises multi level encryption...
That is insanely fake and dangerous