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.
Actually, it is not so surprising they have got over 5 million downloads, considering the age and reputation of that wallet. If you check carefully the creator of that wallet (Andreas Schildbach) has an account here on the forum and is even a moderator on one of the sub-sections of this forum (the Android wallet child-board).
Also, that wallet is featured on the wallet scrunity website and last time I checked, it has a spotless track record of being reproducible/open source.
Yes, the wallet actually displays the fee before sending the transaction and has options to display funds on BTC, mBTC, MicroBTC and Satoshis.
Again, it is not a perfect wallet, but I like it .