'As it happens, I'm aware of that' is similar to 'you may not realise this, but I am aware of that'. It gives an air of emphasis that the speaker is telling the other person something that they don't think they know. It can be used in situations where there is some element of coincidence - for example if you met someone and found that you knew someone in common unexpectedly, you might say 'As it happens I know your brother. I used to go to University with him in Heidelburg', or something like that. Another example might be if someone told you they played basketball and you might say 'As it happens I play myself'. It's difficult to give it any definite meaning as it's sort of joining words that don't really convey any definite additional meaning.