Well, I don't have a android device or nissan hands-free, but I do use a Touch Pro 2 with my sync equipped ford. Anyway, I had a very similar issue, and I had to go into the bluetooth manager and change a setting that had it default to audio over the hands-free device, rather than some other thing... It has worked great ever since then. Oh, I also learned that I had to answer the call using only the car's answer buttons otherwise if I hit answer on the device it would again send the audio only to the phone's speakers. Of course that seems logical, but my phone prior to that would allow me to answer and speak through the hands-free with either method of answering the phone.
At least it's something to look into.
Also, the phone I had before the TP2 required that I update my sync software before it would work. However, it wouldn't even pair before the update, so I don't think that's your issue. I don't know what the procedure is for updating your nissan firmware, but sync simply involves downloading a system version checker to a usb drive, plugging it into the car, return to computer and download available updates, and then return to car to install updates. I would guess nissan would have a similar method?
I realize we're dealing with two completely different systems here, but I figured I would chime in anyway since I had a very similar situation.