It's very impressive that this wallet has had over 5 million downloads so far. Unless the numbers on Play Store are fake and they can somehow be manipulated and inflated. But I don't think that's what is happening here.

I checked the screenshots on Play Store, and I came across this one:



Judging by the text that appears just above the PIN field when creating a transaction, the user is informed how much will be spent for the mining fees. In this particular example it's 0.01 mBTC.