Just updated to latest version (2.2) and saw the checkboxes that you can check in the properties window. There is three different options to choose from which two is pretty self explanatory, the checkbox that is checked and the one with minus in it. But what does the blank one mean? I tried all three and didn't notice any difference between the blank and minus. Probably not only specific to android studio but first time I se it.
It might be a bit late for an answer, but I've just had the same question while playing around with Android Studio. The meaning of the three states of the check boxes for boolean properties can be found by looking at the text view of the layout xml-file: