-1 stands for "no-change".
onSynchronizationUpdate could handle a) progress, b) current downloaded block, c) total amount of blocks.
So the progress could show more informations (current block / total blocks).

But, yes. The third param (total amount of block) is never touched. That's because i took this pull out of a hardly changed BitcoinManager.java.
Let me also integrate the peer updated and then it makes more sense.