Exchange Rate Converter - Android/Web API

In my opinion, the question below contains the most comprehensive answers in terms of figuring out where to get live exchange rate data:

How do I get currency exchange rates via an API such as Google Finance?

Unfortunately, this question was asked a few years ago.

Currently, I believe that the European Central Bank (ECB) data is the most convenient source of data which one can use (even for commercial purposes) without legal implications.

Now, my question is, are there any other free and commercially usable sources of exchange rate data?

I understand that Yahoo and Google data may not be used commercially. Am I correct in saying that if I have an app which uses exchange rates to achieve a certain purpose (ex. convert prices), I can't use Y! and Google?

Thanks.